I never considered how her friends knew the date of her birthday without conversing her. However, they might have learnt about it from another source or they had only touched on it briefly and Zoey never took the time with the group to have a longer conversation about it, whereas with her roommate she has plenty of time every evening and morning to discuss whatever.
Zoey doesn't appear to have any fond birthday/Christmas memories, she certainly never touches on any during the books and since her best friend did die (or was perceived to) less than a month prior, so I'm not surprised that topic might not have come up before everyone bought their presents. Zoey certainly had more important things to occupy her mind: an undead best friend, an evil mentor, a jealous boyfriend, a clingy ex-boyfriend, a seductive teacher, Dark Daughter duties, and homework.
Personally, I put all the blame on her friends because I just don't understand why anyone would think those "Birthmas" themed presents were a good idea.