Well, I guess, considering the fact that fans celebrate March 10[sup]th[/sup] as Mario [Mar10] Day, that May 4[sup]th[/sup] is "May the Fourth be with you" day and May 5[sup]th[/sup] is the "Revenge of the Fifth"--the Zelda franchise could probably get its own fan day worth celebrating the series. I usually just keep in mind the dates I beat individual Legend of Zelda games and celebrate my anniversaries.
There are days for almost everything nowadays, though half of them aren't even official. What I mean by that if the word is spread around well enough, it's an unofficial holiday, right? The three fan holidays I mentioned above are technically unofficial holidays, too. You are not going to see that printed on a calendar when you first purchase it. But sure, if you select a day and make legitimate reasoning behind why that day was chosen, with the internet's magical ability to spread news quickly, you probably could make a Zelda Day if you wanted to. --Even if it's just amongst the DGN community, we could probably arrange something. For example, look what our fellow member, Ghirahimiscool did with Kybyrian Day. We celebrated, right?
But, if we're going to have a Zelda Day, we have to celebrate other series, too!