The line of reasoning is that since those in the Bible celebrated weddings, it's okay to celebrate the anniversary of the wedding. The logical fallacy is that Jesus' birth was celebrated, and Witnesses continue to celebrate births through baby showers, etc. So why not mark the anniversary of one's birth?
It's really sad to see these JW Chirstmases held on married couples' anniversaries, where the whole family exchanges gifts. And the "potty training parties." Just celebrate frickin' birthdays already. But God's people didn't do it? Herod, despite being characterized as a monster, was still debateably one of "God's people," and as far as Pharoah's birthday, the Isrealites who wrote and read the account were sure familiar with the custom, weren't they?