The Possessive Nature of Holidays . . .
Valentine's Day - with the possessive "s" - is the way it is because the day "belongs" to your lover or the one you love (the grammar on that one always confuses me). You mark it accordingly.
Presidents Day - with the boring "s" that just makes something plural - is there because the day honors all of our Presidents (don't get confused by the fact that the holiday is officially George Washington's Birthday (the possessive because it was, well, HIS birthday). It is a day FOR them . . . not OF them.
Want to test the theory? St. Patrick's Day (with the "'s") belongs to Catholics, the Irish, and want-to-be-day-drinkers of all nationalities and backgrounds. Halloween (with no s at all) is a day no one wants to claim and that offers no real reason to fight over. We don't worry about it. New Year's Eve/Day? It belongs to the beginning. Bachelor's Party? No. Bachelor Party. That is an event more for the horny, dried-up, and frustrated men the groom considers friends and family than him anyway so - no need to apostrophize (that is not even maybe a word).
Now you know. Enjoy the day. Buy some towels or a mattress. Buy clearance candy from Saturday's overly-commercialized non-holiday. Thank Rutherford B. Hayes for all he did for this land that belongs to you and me.