We don’t know who Satoshi is and we likely never will, but the pseudonymous founder does apparently have a birthday. Satoshi entered a birth date when they registered the pseudonym with The P2P Foundation. Satoshi’s birthday is, according to that registration, April 5, 1975. (For proof, Satoshi’s age showed up as 38 on their The P2P Foundation on April 4, 2014 and as 40 on April 5, 2015, implying an April 5, 1975 birthday.)