There are numerous problems with connecting IRL stuff with in-game stuff, but I would say that the largest and deadliest pitfall would be that it breaks all immersion. In order to do it right, you need to be very careful, but it should be possible with good planning.
Think of a TV-show. Now imagine that this show, in only a single episode, breaks the fourth wall. This is essentially what you're doing. In order to keep the immersion tight, a bridge between real life and the game is needed. This could be a magic portal that lets stuff travel between the two realms, but perhaps easier and more appropriate would be a powerful character. In StarTrek there's Q, in Supernatural there's Loki, in Warehouse 13 there's a TV that sucks people into its own reality and so on.
Pitfall #1: Gimmicky expectations
In order to not destroy the campaign, make it clear that this is a one-off, not a thing to expect regularly. You don't want your players to think that the campaign is now a gimmicky campaign or that breaking the fourth wall will be a common theme from now on. In a worst case scenario, this could end the campaign.
More likely though is that the players will become primed to include IRL-information when considering puzzles. This could add confusion, non-active gametime, metagaming and non-immersion to the whole campaign.
Pitfall #2: The players don't get it
Normally, I would never start guessing riddles based on OOC information so unless it is made blatantly clear (that is, spelled out in plain text) for me I would never even start thinking in that direction. You can either have the players gain this knowledge by letting someone tell it to them in-game or simple tell them straight out yourself. But be clear, be abundantly clear, that the players will need OOC knowledge to solve the problem.
Also, make sure that the solution actually tells the players something - that the players can figure out that it's all about their birthdays. And give them a present in-game when they solve it.
Pitfall #3: It adds nothing
An event like this has to add something to the game or it will just be a lot of careful planning for a "meh" response. Give them a present/treat in-game when they solve the riddle, and preferably one IRL too. Don't let the session be just another session, but make it special. You could throw in an old NPC the players like or create a new, memorable one.
Most importantly, keep the spirit up. You're connecting this adventure to their birthdays and that should be a fun thing.