When is Harry Potter's birthday?

Question

Can we determine the boy wizard's birthday?

Answer 1

Harry’s birthday is July 31, 1980.

We get the year from a feature on JK Rowling’s old website, Wizard of the Month. Although the site is down, the HP Lexicon got images from every entry. Harry’s tells us he was born in 1980:

The books establish that Harry’s birthday falls at the end of July. He actually shares a birthday with JK Rowling. In Harry Potter and the Sorcerer’s Stone, at the end of first week at Hogwarts, when Harry meets Hagrid, he tries to talk to him about the Gringotts break-in, saying that it happened on the same day as his birthday I.e. July 31. I also found confirmation from JKR herself, in a tweet earlier this year:

For example, on their birthday this year, she said on Twitter:

Thank you so much to everyone wishing Harry and me a happy birthday! I'm having a wonderful time in the sun with family and friends. xxxxx

— @JK_Rowling on Twitter, 31 July 2015

This doesn’t always match the dates in the book – Harry’s birthday often falls on the wrong day – but JK Rowling has acknowledged on multiple occasions that her maths and dates are quite dodgy.

Answer 2

July 31, 1980

Or depending on the book you look at

July 31, 1979

First the day and month:

While it’s a well known fact for Harry Potter enthusiasts, most people don’t know this. J.K. Rowling, the billionaire authoress of Harry Potter, was born on July 31st, 1965 near Bristol England. Rowling decided an appropriate date for the birthday of her main character was the same as her own¹.

₁^{JKR Birthday}

The year is a wee bit more difficult:

Harry Potter and the Sorcerer's Stone (pg 32) it says that "tomorrow, Tuesday, was Harry's eleventh birthday." July 31 doesn't fall on a Tuesday very often. Most readers of that first book assumed that, because it was published in 1997, Harry attended Hogwarts during the 1990s. In 1990, July 31 fell on a Tuesday. This would mean that Harry was born in 1979.

But hold on!

In Harry Potter and the Chamber of Secrets, in which Harry helped celebrate Nearly Headless Nick's deathday anniversary on October 31, 1992. Harry was 12 at that time. So this would mean that Harry was born in 1980.

Source

Most places use 1980, it's just much cooler to say!