From jwfacts.com
https://www.jwfacts.com/watchtower/celebrations.php
God's Kingdom Rules! makes it appear that birthdays were identified as unacceptable in 1936, by quoting from The Golden Age of that year.
""There are but two such celebrations mentioned in the Scriptures, one of the heathen king Pharaoh of Egypt, in the days of Joseph, and the other of [Herod,] whose birthday cost John the Baptist his life. In the Bible there is no instance of celebrations of birthdays by any of God’s people."—The Golden Age, May 6, 1936, page 499." God's Kingdom Rules! (2014) p.105
In reality, birthdays continued being celebrated until 1951. In 1940, the following experience shows birthdays were still considered appropriate.
"Pardon me for intruding on your precious time, but I just can't help letting you know how much I appreciate the phonograph which came to me on the morning after the 8th, which was my 80th birthday. It was indeed a birthday gift from Jehovah, to be used in proclaiming his name. May grace and strength be given me to do with my might what my hands find to do." Watchtower 1940 Jan 1 p.16
In 1951, that the Watchtower stated the Witnesses were no longer to celebrate birthdays.
"Is it proper to have or attend celebrations of birthday anniversaries?-F. K., Nevada.
Such celebrations have their roots in pagan religions, and not Scriptural grounds. Some Bible commentators suggest that birthday celebrations may have had their origin in the "notion of the immortality of the soul"." Watchtower 1951 Oct 1 p.607 Questions from Readers
There is nothing unscriptural about celebrating birthdays. (See Birthdays for detailed information on why the Watchtower ruling against birthdays is wrong.) Most Witnesses struggle to explain why they can't without some embarrassment at the shallow reasoning used. Romans 14:5-6 advises not to judge others and their celebrations:
"One indeed judges a day above another; and another one judges every day alike, Let each one be fully assured in his own mind. The one minding the day, he minds it to the Lord. And the one not minding the day, he does not mind it to the Lord. The one eating, he eats to the Lord; for he give thanks to God. And the one not eating, he does not eat to the Lord, and gives thanks to God."