Despite the Handbook series authors' occasional attempts to tell you otherwise, in the Inner Sphere culture is determined more on a planet-by-planet basis than a State by State basis. (I'd much more strongly recommend the FASA era House books for information about the Successor States) Whether any given Holiday is celebrated is best answered by beginning with who colonized the planet and then making plausible assumptions from there.
Take Algedi as an example. Settled by a (fictional) sect of Muslims largely from North Africa. Will they celebrate Christian holidays like Christmas and Easter on Alegdi? Probably not. Since the Azami sect is fictional, you can't say what sorts of views they have on real world holidays, but you can probably make some assumptions that Islamic holidays like Ramadan are observed.
Contrast that with Luthien in the same star empire. Lutheranism has a sizeable minority in the Imperial City, and so Christmas is actually a big deal on Luthien. Even though it largely isn't across much of the rest of the Combine.
Another fun WTF for "Space Japan" is that there are enough Hispanics on enough worlds that
Quinceañeras are commonly observed in the Draconis Combine, even by non-Hispanics.
Also consider that "Space America" was the Terran Hegemony, and many of the worlds that would be most similar to cultures (most of us) are familiar with are either dead or have been battlegrounds for centuries.
