Birthdays are not a disfellowshipping offence. However most elders are unaware of this and may d/f you for it. It is not listed in the elders book as such. There is an article, but I can not find it, that shows that birthdays are a conscience matter, though birthdays are generally worded that a mature Christian would not participate.
w199810/15p.31QuestionsFromReaders "Consequently, while it is entirely a private matter if Christians choose to take note of their wedding anniversary, there are good reasons why mature Christians abstain from celebrating birthdays."
Another interesting article is on Pinata's, showing such things can be a conscience matter.
g03 9/22 pp. 23-24 The Piñata?An Ancient Tradition *** The Piñata Today "Later, the piñata became part of the festivities of the posadas during the Christmas season and continues as such to this day. (A star-shaped piñata is used to represent the star that guided the astrologers to Bethlehem.) Breaking the piñata is also considered indispensable at birthday parties. Indeed, piñatas have become so traditionally Mexican that Mexico even exports them to other countries. We found that for many people in Mexico, the piñata has lost its religious significance and is considered by most to be just harmless fun. In fact, piñatas are used in Mexico on many festive occasions, not just for the posadas or for birthdays. And piñatas can be purchased in many forms other than the traditional star shape. They are sometimes made to resemble animals, flowers, clowns. When considering whether to include a piñata at a social gathering, Christians should be sensitive to the consciences of others. (1 Corinthians 10:31-33) A main concern is, not what the practice meant hundreds of years ago, but how it is viewed today in your area. Understandably, opinions may vary from one place to another. Hence, it is wise to avoid turning such matters into big issues. The Bible says: "Let each one keep seeking, not his own advantage, but that of the other person."?1 Corinthians 10:24."
I would recommend that if you do not want to be d/f say you thought is was a conscience matter and play innocent. The elders disfellowship on your attitude, and can d/f you for anything, or let you off anything, depending on how much you are prepared to grovel.