I like to have New Years Eve be at the last day in Winter, which puts Valentine's Day in Spring and though I know that doesn't correspond with RL, the simple reality is that the entire Sims calendar is not a Julian Calendar so the specifics of what Holiday happen in what season is whatever the heck I want them to be.
Keep in mind that even in the real world Seasons and Holidays are not exactly the same. Christmas happens in Summer, not Winter, for half of the planet. Easter happens in Fall, not Spring, for that same half of the planet. And for some people New Years was just a few days ago rather than over a month ago.