Just want to share what I've uncovered on this and verify my conclusion!
It was my impression that birthdays were banned shortly after Christmas was in 1926. The Proclaimers book is very vague about this, implying that the ban shortly followed the Christmas ban.
I searched the 1940s Watchtower magazines and uncovered this blurb in the Jan 1, 1940 Watchtower, p. 16, under "Field Experiences":
CLEVELAND, OHIO: "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 proclaming his name. May grace and strength be given me to do with my might what my hands find to do."
There is no prohibition of birthdays mentioned at all in the 1940s Watchtowers, aside from a quick negative comment within a discussion on Christmas about how the Early Church Father Origen mentioned that only sinners celebrated them in the scriptures (Nov 15, 1942, p. 349).
The next mention of a birthday celebration in the Watchtower Magazine comes in 1951:
*** w51 10/1 p. 607 Questions from Readers ***
Questions from Readers
• 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". Astrologers and stargazers laid great stress on offering sacrifices to the gods each year when the stars and planets were in the same position as when one was born. In Egyptian mythology the "birthdays of the gods" were celebrated on certain days, and in Chinese mythology individuals offered special sacrifices on their birthdays to Shou Hsing, the god of longevity. The ancient Anglo-Saxons celebrated the birthday of the "Lord Moon", spoken of as meni at Isaiah 65:11 (margin), by making cakes "called Nur-Cakes, or Birthcakes"; and candles also are of pagan origin.—See Hislop’s Two Babylons, pages 95, 191-196.
After telling us that December 25 was the traditional birthday of Nimrod, and not of Jesus, the new book What Has Religion Done for Mankind? states: "The inspired Scriptures do not give the birth date of Jesus, and it does not matter, for neither Jesus nor God his Father nor the inspired apostles instructed us to celebrate Jesus’ birthday. The only birthday celebrations that the Holy Scriptures mention are those of pagans, those of Egypt’s Pharaoh and of Herod Antipas who marked his birthday by having John the Baptist’s head chopped off. (Gen. 40:20; Matt. 14:6; Mark 6:21) Christ’s disciples of the first century shunned birthday celebrations as being pagan, unchristian!"
Doubtless many things practiced by Christians today were also practiced by pagans; but when these practices are steeped in false worship contrary to Bible principle they become objectionable. The celebration of birthday anniversaries centers the mind on the creature and exalts the creature, giving him and his birth undue importance. Romans 1:25 (NW) warns of those who "venerated and rendered sacred service to the creation rather than the One who created". Birthday celebrations could tend to take on this objectionable quality. If Christians wish to come together occasionally for profitable fellowship and relaxation, they do not have to await a day reminiscent of pagan religion. If they wish to present a brother with a gift, they do not have to await the anniversary of the day of his entry into the world, as though that were such a memorable occasion. If the precise day of Jesus’ birth and its remembrance were of no such noteworthiness, whose are?
So I assume this 1951 QFR was the "new light" that officially banned birthday celebrations. Am I correct?