Of course, strip time is a little suspect. Monica said this year that she was 31. If strip time and real time are supposed to be linked, that means she was working as a researcher at the museum since she was 18 (and not getting carded at the bars).
I always assume no connection between strip time and real time.
One webcomic I read began in late 2010 at the beginning of the main character's senior year of high school (well, there was a three-page prologue set roughly 13 years earlier) - so, in-comic, late August or early September. There have been at least two new pages almost every week, and it is now January of the same school year. The comic has very carefully avoided stating what the in-comic calendar year is, but I have reason to believe it's 2002 (2007 is also possible).
Another webcomic once - explicitly - had
four pages covering less than one second of conversation between two characters, but the 2nd page after that has one panel consisting of the words "six months later". (This comic, by the way, began in January 2002 with the main characters being juniors in high school. They are now seniors in high school. In the real world, people who were in first grade in January 2002 have already graduated from high school.)
With calendar disparities like that, it is inevitable that both real-world holidays and story-world holidays will occur at times that don't match the other world. So holiday snapshots such as these, matching real-world holidays but with no stated correspondence to story-world events, I regard as not part of the story. Just fun things the artist decided to do and share.