By using seasonal songs, you're reinforcing the current season, orienting the child to the time of year (and the concept of a calendar), introducing the life cycle, and reinforcing the fact that they adapt to seasons (ie: clothes, activities), whether or not they realize it.
10 Songwriting Tips for Music Therapists & Music Educators
Kid-Friendly Valentine's Day Songs
Fun Christmas Music for Kids
Ahhh... holiday music. I'm a sucker for the classics. A remake rarely beats out the original recording. (Snoop Dogg Blue Christmas, anyone? Oy vey.)