In the story "Charmed Strife" from Scooby-Doo! Mystery Comics #21 (October 1973) reveals the gang's birth signs to be Shaggy as a Leo (July 23 - August 22), Fred as a Pisces (February 19 - March 20), Velma as a Virgo (August 23 - September 22), Daphne as a Gemini (May 21 - June 20) and Scooby as an Aries (March 21 - April 19).
Scooby also reveals he was born in April later on in the comic.
Interestingly, in the episode "Happy Birthday, Scooby-Doo" (an episode centered around Scooby's birthday), Scooby holds up a calendar which mentions his birthday is on Saturday the 7th. Coincidentally in 1984, the year the episode aired, April 7 was on a Saturday, which aligns perfectly with what the comic states his birthday is.
Credit for finding all the images as well as noticing this fun fact goes to Samuel Kirkwood.