I know it's not "officially" summer yet, but it's Memorial Day weekend and that's just as good.
I remember watching Ed Hurst's "Summertime At The Pier," a Bandstand-type local dance show that broadcast live from Atlantic City's Steel Pier. "Be My Baby" was playing as they showed live shots of the surfers, and the song became, to me, indelibly synonymous with summer.
What song automatically says summer to you?