英文字的文法難題？

Hyphens are normally used in between two nouns when there is a possibility that there could be a confusion over the meaning of the sentence.

E.g. I want to eat ice cream. <---In this case, we should put hyphens between since both ice or cream can be consumed, to avoid the confusion, we should use ice-cream instead.

In "sales office", we are "sales" and "office" are not similar things, so no one would be confused by this, so "sales office" would be sufficient.

Similarly for "moon festival", "moon" and "festival" are unrelated nouns, so it is not likely anyone would be mistaken by this with some other meaning, so the "moon festival" is perfectly fine.