Hot Summer Day by "It's a Beautiful Day", one of the the old San Francisco psychedelic era. IMHO, their first album "It's a Beautiful Day" was a classic; they're second and any subsequent, not so much.
It's the only live version I could find of "Hot Summer Day", and it's recent but back in the day, they were hotties. Here they are back in the old days, singing their big hit "White Bird." Try to ignore Bill Graham's blather, if you can.
I also ran into this site with a list of the "50 Greatest Summer Songs."