It seems Scooby Goes Hollywood may not have premiered on December 13, 1979 like we all thought! A reference book titled Animated TV Specials: The Complete Directory to the First Twenty-Five Years, 1962-1987 lists the special as airing December 23, 1979. This book pre-dates any Internet records, as it was published in 1989. On top of this, a TV guide listing site lists Scooby Goes Hollywood as airing December 23, 1979 at 7:00pm. I couldn't find any evidence to support that December 13, 1979 other than what other non-historical sites have posted, meaning it's possible that somewhere along the line, somebody confused the dates (maybe even by accidentally typing a '1' instead of a '2') , and more and more sites posted that incorrect information until it became widely accepted.
Granted, I suppose none of this is official "fact" since it's just speculation, but I thought it was super interesting and wanted to share. Thanks so much to James for researching this fun fact!