We have a Word of God statement that there is at least one Jewish wizard at Hogwarts, Anthony Goldstein (it's in an answer here somewhere). But if so, why are only the Christian holidays celebrated? Shouldn't they either celebrate all holidays or none?
I don't think it's really as complicated as some of the other answers (and comments) suggest. The British, by and large, consider the secular celebration of Christmas to be part of their cultural tradition; the fact that some people celebrate it as a religious holiday has always been perfectly acceptable, of course, but was not traditionally considered a matter of any particular concern. (Live and let live, it would be a funny old world if we were all the same, and so on.)
The idea that celebrating Christmas (in a secular fashion) is in any way unfair to people of non-Christian faiths has I suspect to some extent been imported from US culture in recent years, but to the best of my knowledge was not at all common during Harry's time at Hogwarts even amongst Muggles.
Add to this that the wizarding community generally lags behind Muggle society on social issues in general, and consider that it isn't at all clear whether the wizarding community is on the whole even aware of the existence of Muggle religions, and the fact that Hogwarts celebrates Christmas really is entirely unsurprising.
Additional: note that I am confident that students of other faiths would not be obliged to take part in or even attend the celebrations, at least not while Dumbledore was in charge. (The Death Eaters, on the other hand...)
There are three main elements here.
are were no Jewish students at Hogwarts.
Bear with me before you start yelling. One day, JKR waved her magic twitter-wand (long, long after she'd finished writing the books) and deemed one of the students to be Jewish. Since there are no Jewish students mentioned in the books, there's no need for the whole school to celebrate any Jewish holidays.
UK Boarding schools don't observe non-Christian holidays.
Hogwarts is modeled on JKR's understanding of what a UK boarding school is like. The vast and overwhelming number of famous boarding schools only celebrate Christian holidays, indeed even their calendars and term-dates (Lent term, Michelmas Term, Trinity) are based around those same holidays, long after anyone actually observes them.
UK schools (in general) don't usually observe non-Christian holidays
In the UK, it would be unheard-of to have a 'whole school observation' of a non-Christian holiday unless students of that religion were in the majority within that school.
Out of universe
J. K. Rowling is a Christian and as thus she wrote the books about Christianity.
To me the religious parallels have always been obvious, but I never wanted to talk too openly about it because I thought it might show people who just wanted the story where we were going.
Harry Potter' Author J.K. Rowling Opens Up About Books' Christian Imagery - Open Book Tour
She has said (under oath) that she never had plans to write any books about Judaism.
Defense: You were never going to write a book on Harry Potter and Judaism, right?
The Court: And what?
Defense: It was never her intention to write a book about Harry Potter and the Talmud, isn't that correct?
Rowling: I regret to say that is the case.
Warner Bros vs RDR Books - Day 1 - J K Rowling's cross examination
Also, as others have pointed out, this was the culture norm for schools in the UK.
There are a few possibilities here.
1. They do celebrate Jewish and other religions' holidays, but it is not mentioned in the books. Since I assume Harry, Ron, Hermione, Neville, and the majority of the students in Hogwarts (as well as, of course, JKR herself) are Christian, or at least they do not celebrate or observe non-Christian holidays, it would make sense that none of them ever notice these holidays, and so they could have easily been cut from the book. Especially since none of the, matter towards the plot.
2. Majority rule Most, if not all of the faculty is Christian in some way, shape, or form, as well as almost all of the student population. There could just be not sufficient numbers of other religions to have any say in this.
3. Hogwarts was a previously Christian school Many prep-schools in America, at least, were previously all-Christian, so it could be possible for Hogwarts, especially regarding that it was founded in the Middle Ages.
However, the real reason is that JKR didn't write about them. She probably did not know much about them, and just didn't put them in.
Surely the answer is simply that Hogwarts is an English school, and a quintessentially anachronistic English boarding school stereotype at that, and traditionally such schools are nominally Christian and really do only celebrate Christian holidays - the entire English school term system is based around the church calendar.
Or to put it another way...
Hogwarts is an old-fashioned English boarding school. This is what it's actually like in old-fashioned English boarding schools in the real world. At the time JKR was at school, I very much doubt any English school didn't celebrate the Christian festivals or did celebrate other faiths at all.
First of all, Easter isn't really celebrated. Students get it off and are allowed to go home, but there's never a mention of a feast or decorations or the like. Harry gets eggs from Mrs. Weasley, but that's entirely private.
Hogwarts does get decked out for Christmas and Halloween, and has a feast for each. It also gets decked out once for St. Valentine's Day. But all these holidays are widely celebrated by non-Christians (to the extent that few even really think of the latter two as Christian at all).
So I think it's really just a matter of practicality: huge numbers of your student body are going to be celebrating Christmas and very few will be celebrating Diwali. It just makes sense to have schoolwide celebration for one but not the other.
I suppose they could celebrate NO holidays, but you also have to consider the importance of tradition. For the thousand or so years of Hogwarts' existence, 99% of their student body would be Christian up until very, very recently. That means for a thousand or so years, you'd get holiday on Easter and Christmas and a feast for Christmas. Who wants to be the headmaster who scraps the Christmas feast for sensitivity's sake?
I went to school in England in the time frame that the books are set in. I didn't go to a boarding school, let alone a magical one in a remote part of Scotland, but I was in Cambridge which has a high density of fee-paying boarding schools and their terms were much the same as ours (if slightly shorter).
The whole academic year in the UK is structured around three big holidays: Christmas, Easter and summer (the longest one). Hogwarts follows this, possibly because it makes sense in terms of fitting in with the Muggles (wizarding kids would be home from boarding school for the holidays at times which suit the expectations of the Muggle neighbours and thus reduce suspicion).
The reasons for the UK's school system stem of course from Christianity, which is the established religion in England and the religion of the majority of the country who hold a religion, and more so in the time of the books than today. These days we're one of the most atheist countries on the planet (even if people say they're Christians, observance and actual belief are pretty low), but we still celebrate Christmas as a big winter festival - with many elements everyone enjoys these days (including those of other religions) being those adopted from the midwinter festivals preceding the arrival of Christianity which were borrowed as a framework for the celebration of Christ's birth (just like the spring/fertility festivals became Easter, and we retain that imagery in Easter eggs, chicks etc.)
The summer holiday is from farming, giving the children time away from school to help with the harvest and all the massive activities required on a farm during those most productive months of the year in the northern hemisphere.
So all Hogwarts is doing here is expressing the UK's primary cultural heritage. There are clearly students present from other religious backgrounds, but there's no fuss made in the text about anybody actually observing any religions - none of the students who might be from Christian families ever mention going to services or praying or anything like that, and we don't see the Patils (who we might guess are Muslims descended from immigrants from Pakistan) praying five times a day. Rowling seems to have decided to just not mention that side of life in the books - Harry clearly isn't religious, so maybe it's just not important to his worldview and so doesn't get mentioned from his POV. Or the books just reflect the nature of day to day life in the UK - if you're not actually religious yourself - which the majority of us aren't - you mostly just don't think about it.
Many modern schools in our sad, sad Muggle world don't expressly focus on one religion more than the other, be they public or private. When they do, it's part of the school's founding or heritage, if there's a stark majority of one religion, or if the school's admins deem it so. However, even completely secular schools in the "West" have holidays and traditions around Christianity--winter break near Christmas, spring around Easter. In other areas of the world, spring/winter/summer breaks coincide with other cultural or religious holidays. For example, the weekend in a some Muslim countries is Friday-Saturday, not Saturday-Sunday, because Friday is the holy day of the week in Islam.
Hogwarts was founded
"A thousand years or more ago, when [the Sorting Hat] was newly sewn"
-Harry Potter and the Socerer's Stone
At that time, the UK was (and I believe still is?) a Christian-majority nation. Practitioners of other religions were often persecuted or isolated by choice. There were also other factors of population/religion shift such as during the plague of Black Death in the mid 1300s. It killed off a huge percent of the European population, sparking a surge of zealous Christianity/religiousness. Furthermore, thousands of Jews were blamed for the pandemic and more or less kicked out of the area.
Because the world is a vast place and it does not observe cultural traditions or expectations of the United States, even in a relatively similar environment as Western Europe.
The modern USA population consists mostly of descendants fleeing not only from poverty, but also from religious persecution in Europe. That caused the immigrants to look out for any possible prohibition or even religious slights. This caused the strong insistence of a secular country. The own belief is still a marker of an ingroup and discussing religion is still undesirable.
In the meantime the Europeans got less and less religious observant at all. Religion was considered less and less important and it simply ceased to be an ingroup marker. I think many people here who live in Europe have for most of their acquaintances no idea which religious denomination they have. Nobody also cares for religious observances anymore: It is not uncommon to have people calling themselves "Christians" who eat beef on Fridays and add own personal beliefs which are incompatible with the official declarations like the belief in reincarnation etc. Atheism is widespread, atheists are even the majority in Scandinavia.
While religion lost mostly its power, strangely it is still considered a bringer of virtue. During the fall of the East German regime people used churches as meeting points and some pastors like Martin Gutzeit were spearheads of the resistance. Many churches are still greatly involved in charities and care.
And this is what is very, very hard to understand from a US perspective: In some parts of Europe there are church taxes, the government collects taxes for the main Christian churches. There is also in some parts obligatory religious education in state schools, yes, you read that right, pupils are going into school and are teached Christian knowledge which will be also graded. And everywhere people are celebrating Christian holidays because of tradition and they do not care a damn if this favors the Christian denomination.
If you do not want to pay taxes, officially declare to leave the church. If you are not part of a minority which is automatically excluded from religious education (Jews, Muslims), you talk with the principal and find a solution. But it is the decision of yourself and your parents. There is really not the viewpoint all or none, this conformism does not make sense for Europeans in general. If you want to celebrate, celebrate, if not, do not.
So the answer of the question is: Because Europeans in general observe Christian holidays for tradition, find Christian traditions good and ignore if there are other religious holidays.