1927 - Celebration of Christmas and birthdays banned
Birthdays were not expressly banned in writing until 1951.
There are a few accounts in the Watchtower Magazine of the 1940s of Witnesses taking part in birthday celebrations, as well as an account in the 1946 yearbook:
http://www.jehovahs-witness.com/12/106913/1.ashx
Although it appears that in some JW circles at least the practice was frowned upon prior to 1951.