Sonic's Birthday
Sonic the Hedgehog came out on June 23rd, 1991. Many people consider this day his "birth date." Since then, Sonic's birthday HAS been celebrated.
It's a boy! Sonic The Hedgehog - The one that started it all. Sonic the Hedgehog is technically Sonic's birth.
Happy 6th Birthday, Sonic! Sonic Jam - Sonic Jam could technically be called Sonic's 6th birthday. It pays a homage to Sonic's then-famous Sega Genesis games, and even features exclusive artwork. Though it isn't exactly a Sonic game, the Sonic World feature in it is quite a feat. Sonic Jam would be a predecessor to Sonic Adventure.
Happy 10th Birthday, Sonic! Sonic Adventure 2 - Sonic Adventure 2 is explicitly Sonic's 10th anniversary, even featuring a 10th anniversary set that was released …