Ecclesiastes 3:12
Ecclesiastes 3:1212I know that there is nothing better for them than to rejoice and do good while they live,
The Watchtower cites — "I have come to know that there is nothing better for them than to rejoice and to do good during their life" — as permission for "wholesome gatherings" of family and friends other than birthdays. JWs hold weddings, anniversaries, graduations, and congregational social events without difficulty; only birthdays specifically are forbidden (Reasoning From the Scriptures pp. 68-70).
blesses joy and doing good across the whole of life, without qualification. The Watchtower permits celebratory gatherings generally and then singles out one day each year — the day a person was born — and forbids only that. The asymmetry is the problem. ("cursed be the day I was born") and ("Job cursed his day") both presuppose that the day of birth was normally blessed — you cannot wish a day unblessed unless blessing it was the custom. prophesies that at John the Baptist's birth "many will rejoice." is perfectly at home with all three. And — even in the NWT — explicitly permits a Christian to observe a day "to Jehovah" while another Christian treats every day alike, with both acceptable. 's "rejoice and do good" covers ordinary family celebration of ordinary human milestones, including birth. The Watchtower's specific birthday ban is a rule added where Scripture does not add it.