I'm not sure how to write the following senteces: Moon Festival and Sun Festival.
I have done my research on the words, though:

Gealach = Moon
Grian = Sun
Féile = Festival

Should I write "Gealach/Grian Féile" or "Féile Gealach/Grian"? Or are there any words that must come in between, such as the Irish equivalent of "of" and "the"? I've tried to find out by myself but I couldn't find anything conclusive.

Thank you.

It kind of depends on what you mean. Féile general is a religious feast day (like tomorrow). English festival has a wider meaning, for instance the town I grew up in had a "Grape Harvest Festival". It was more of a fair than a feast day celebration. Also the word which you eventually will use for "festival" will be placed first.

Yeah, féile is derived from Latin vigilia and I daresay would be both anachronistic and inappropriate for naming a pagan feast (which I kind of suspect is what you're trying to do). Feis strikes me as a better choice, but it still may not be the best.

Whatever word you go with, "sun" and "moon" are going to have to be in the genitive case. For instance, Feis an Gheimhridh "Winter Festival", where the form of "winter" not in the genitive is geimhreadh. Any decent dictionary of Irish will give the genitive form of each noun in the entry.

"Féile" is also used in a general sense for 'festival'. It is not limited to religious festivals

Yes, I mean it to be sort of a religious feast day. Sorry for not specifying.
So should it be "Féile Grian/Gealach"?

Oh I see, Seánw.
I agree with you, "feis" does look like a better word for what I am trying to do (a pagan feast, indeed). I've read several definitions and they are all in agreement.

I have also looked up the genitive case and I believe it will be like this: "Feis an Gréine" and "Feis an Gealaí".

"Feis na Gréine" (grian is feminine - don't ask!).

"Feis an Gealaí" is fine.

No, it would have to be "Feis na Gealaí" (gealach is also feminine.)

I looked the gender up on focal.ie and still managed to get it arseways. Thanks Abigail.

Why is it "Feis na Gealaí" and not "Feis Ghealaí" (as in oíche ghealaí)? I've always wondered when to use the an/na with the genitive.

I can't think of a natural context where oíche ghealaí would be definite, whereas Feis na Gealaí is definite by its very nature. There's only one moon and only one great feast for it (even if it does recur on an annual basis), but a great many moonlit nights.