I think the May eve day goes back long before LaVey. You are, of course, right on the money, an excuse to party. I have never read any LeVay so I can only take others word for how important his perspective is. Again, you are right in that the day doesn't really matter, it just matters to me because I place some value on it. My own personal weirdness I guess.
IN GIRUM IMUS NOCTE ET CONSUMIMUR IGNI