Peanuts by Charles Schulz for August 28, 1951
Transcript:
Violet and Charlie Brown approach Snoopy sitting on the lawn. The latter's ears shoot up as they present him with a cake and sing,"Happy birthday, dear Snoopy..."<BR><BR> They place the cake on the ground and he sits crying as they conclude, "Happy birthday to yoooooo"<BR><BR> They sit next to Snoopy and Violet says,"You forgot to light the candle, Charlie Brown...."<BR><BR> She looks at the candle and Snoopy licks his lips as Charlie Brown replies,"That's not a candle...that's a wiener!"<BR><BR>
His fictional birthday is in August… August 28 (according to this strip) or August 10 (if you look at 1968). Just because he first appeared October 4, 1950 doesn’t mean he was born then.