As for birthday celebrations, the Jehovah Witnesses are equally clear. Lesson 11 from their official web site states:
The only two birthday celebrations spoken of in the Bible were held by persons who did not worship Jehovah. (Genesis 40:20-22; Mark 6:21, 22, 24-27) The early Christians did not celebrate birthdays. The custom of celebrating birthdays comes from ancient false religions. True Christians give gifts and have good times together at other times duing the year.
Isn't this extremely similar to how HWA convinced us that birthdays were immoral?
The truth is HWA stole this doctrine from the Jehovah's Witnesses.
Originally the Watchtower Society allowed birthdays, but it was only under Rutherford, around the 1920s or so, that the Watchtower Society added this condition for its members.
Clearly HWA became persuaded by their arguments and imposed this doctrine upon his followers.