As usual on Sunday morning...

It being Sunday, of course, this is running in Rico's head:

Sunday, Sunday, Sunday! Amphetamine Motor Speedway! Sunday!
See the amazing wheel-standing rapping lunatic Corvair! Sunday!
See Shad Crapper light his nose hairs on fire as he pulsates and blasts down that quarter mile thrillway! Sunday!
See Tim Leary and his Superblown head! Sunday!
See Big Daddy Roth and funny cars from across the universe! Sunday!
12 bucks buys you the whole seat, but you'll only need the edge! Sunday!
Be there!

That's from this guy's site, back in April of 2006, but neither of us can remember where it came from, though he at least tried:

Some Googleage revealed the following: it was probably done by The Phlorescent Leech & Eddie aka Flo & Eddie aka Marc Volman and Howard Kaylan of The Turtles, who, due to a spectacularly badly negotiated contract, couldn't even use their real names as musicians after The Turtles went belly-up, and probably as part of a Frank Zappa album.

Rico says he'd've sworn it was a Firesign Theatre bit, but it seems it was based on reality:

"Jan C. Gabriel's name may not ring a bell, but his voice surely will. It was Jan Gabriel who first came up with the phrase Sunday! Sunday! Sunday! for the US30 Drag Strip and Sante Fe Speedway television & radio commercials. These commercials were arguably the catchiest local commercials ever." 

There were many different parodies, but Rico says none of them are the right one, alas, so he keeps searching...