Some of these don't travel well...
I dunno. Most nations on Earth have some notion of an Independence Day, Founding Day, or some occasion where they celebrate their victory over an existential threat or a monumental achievement of their society. E.g. in France you don't have a holiday called "Independence Day," but you
do have Bastille Day, which is a pretty close analogue, and Germany has Unification Day when East and West Germany were reunited as one nation. You don't have "Thanksgiving" in China or Israel, but you have the Harvest Moon Festival and Sukkot, respectively, both of which are heavily flavored with the same kinds of ideas (the earth's bounty, togetherness with family, thankfulness, reflection, and in the Moon Festival's case, a dessert associated with this holiday). Many cultures throughout human history have had special celebrations of the beginning of spring, and there are often "ghost festivals" or other stuff relating to death and the afterlife in late Summer or early Autumn.
St. Patrick's Day is probably the only one unlikely to have reflections in other cultures, since it's basically "(Catholic) Irish Heritage Day," but even then I could see potential for it when adapted to fantastical settings with a deeply proud culture embedded in them. As an example, in 4e's Points of Light setting, I could see there being a holiday celebrated primarily by dragonborn remembering the heights of lost Arkhosia: eating and drinking traditional fare (which is a bit strong/pungent for most non-dragonborn), engaging in jousting or magical fencing duels (referencing Arkhosian High Style and/or the Ninefold Path), setting off fireworks that resemble dragon-breath or the like, collective wardances, etc. For non-dragonborn, it's an excuse to get smashed on dragondraught, eat fireberry pie and/or flaming fire flakes (hot! Whaddaya know), and maybe snag a dapper swain or charming wench for a night's revelry. Call it something like Golden Dawn, claimed to be the day the Golden One was acclaimed ruler of the seven city-states that formed the empire's heart.
Spring, freedom/unity/solidarity, romantic love, thanksgiving, family love, the afterlife, and national pride are all common motivators. You'd have to tailor them (and the specifics of how they're celebrated), but honestly, it's hard to go wrong with at least the first six of those (and, as noted, you can get pretty creative with the seventh IMO.)
