I'm not sure about this as I haven't read the manga in its entirety (I doubt the manga has a chapter). However, with regards to the anime, they never mention about anyones birthday or even celebrate them. I mean, even between Season 1 and season 2, they should atleast have one episode for that. I doubt Belldandy would pass K1's special day or vice versa... it would also be a great time to have that very long awaited kiss.. I believe this would really be a fun episode to make or create. Manga or anime form. What you think guys? I mean, how could you have a relationship with someone without knowing when they were born right? Just a thought.

K1 birtday is supposed to be in december
Bell birthday in january ( according to the manga in 198 ? )

but for the other characters I think theres no official date

Really? The manga says that? hhhmm.. I read at wikipedia that Bells bday was around January, I think I'll check that Manga chapter out. K1's bday in december? They should have made an episode for that... they made one for Christmas.. oh well, I guess they have their reasons. Still, they should celebrate their bdays. Now that the season 2 has ended (there should, no, must be a season 3) I hope they can make a manga chapter for this.