Panel from Spider-Man Free Comic Book Day 2007 one-shot (May 2007), script by Dan Slott, pencils by Phil Jimenez, inks by Andy Lanning and John Dell, colors by Jeromy Cox, letters by Chris Eliopoulos and Cory Petit
I don't know if this is the first canon reference to her birthday, but of course she could have only been born in May. It all makes perfect sense, just like Spider-Man's mother November Fitzpatrick Parker. I'm just kidding there: Pete's mother's pre-married name was Mary Jane Fitzpatrick. "Mary Jane?" Oh...dear.
Anyway, it's liable to be a hootin' and hollerin' South of the Border flavored celebration taking place on Cinco de Mayo!
So, Happy Mexican Birthday, Aunt May!