In season one, the episode where Ross is upset about the anniversary of his and Carol's first time, the date is October 20. Joey and Chandler walk into Central Perk (the girls are already there with Ross) and say to Ross, "Happy Birthday! We got Rangers tickets" (something to that effect) to which Ross replies, "My birthday was like 6 months ago.

A few seasons later when Joey has to fill out the forms for Ross in the hospital (after Joey tells Ross that he's in love with Rachel), Joey asks him his birthday. Ross says, "You know my birthday" Joey says...Maytember.....Ross then says October 18