May 3rd. This date is derived from “A Friend In Deed” (2×18). Where Pinkie says:
75 Days after February 18th, 2012 (Original Airdate, Leap Year) is May 3rd.
But what about “Party of One” (1×25), where Pinkie says:
The episode aired April 29th, 2011, but since it this surprise party was held the next day during Pinkie Pie’s after-birthday party for Gummy when everypony else couldn’t make it, it would make that April 30th. And we heard the birthday reference straight from the pony’s mouth that day.
SO WHICH IS IT, HASBRO.
(Then you can argue that the exact date doesn’t matter since MLP is a cartoon with little respect to aging and that the HubNetwork Twitter is boss. For the record, Andrea Libman’s birthday is July 19th, so that’s not a reference to that)