From the wikia:
So the chart spells out:
If you accept that the second letter is D. It is not a perfect match, though.
I don't think it has to be a calendar, it could however be a chart. Or a game. Also, the squares are crossed out sporadically, who does that with a calendar(at least your not suppose to). It could also be a map of landmarks or where something is buried.
Alternately this could be impossible to solve if all of the letters in the alphabet aren't released. It would be like a impossible game of hangman.
To supply the rest of the letters, I used a alternate rune calendar and got a bunch of random letters. But this is described through:
From the wikia:
It is interesting to note that, while the alphabet used in the series
is, indeed, runic, some characters are used improperly
So unless you have a complete alphabet, this might be impossible to solve just yet.
The last letter is supposed to be Z in many other tables, but that is not a rule. And the backward F can mean many things.
If you compare to this chart, the rune word's meaning appears to be gibberish.
I even looked through locations and characters it could be referencing to and I couldn't find a match. So I would say that the answer has not been revealed yet.
And it doesn't seem to have anything to do with runic calendars, either.
Rather disappointingly, it could just be something that was stuck on the background as cameo.
And it doesn't seem to follow any other rune alphabet and that could mean that the alphabet terms missing could be anything.
You could try asking a person that knows runes, but the show doesn't follow a set alphabet.
Another assumption I had
I had this assumption while writing this post earlier, but decided not to add it do to the following errors. But after a comment mentioned it again, I decided to add this an answer.
The calendar could be spelling out August. I came to this assumption due to the that "calendar" looks like a calendar and the episode was indeed aired August,7. But the word would be missing a "G".
The first letter is obviously A.
The next letter is unknown.
The next letter could be seen as "U". It does not look exactly like it, so that is why I decided not to assume.
The 4th letter could be seen as "st", but not using the adventure time rune translation, Instead you have to use a alternate chart.
And the last letter using that chart would indeed be x or z.
Anyway you cut it, it wouldn't spell either April or August.
So the word would Spell.
I personally think it probably means nothing. We are looking to much into it.