Pieces that refer to summer, one way or another:
"Sumer is icumen in"
Gustav Mahler, Symphonie 3, I: "Pan Awakens; Summer Marches In"
George Gershwin, "Summertime", from Porgy and Bess
Samuel Barber, Summer Music
Samuel Barber, Knoxville: Summer of 1915
Michael Tippett, The Midsummer Marriage
Felix Mendelssohn, A Midsummer Night's Dream (incidental music)
Benjamin Britten, A Midsummer Night's Dream (opera)
Anton Webern, Im Sommerwind
John Cage, "Summer" from The Seasons
Antonio Vivaldi, "L'Estate", from Le Quattro stagioni
Arnold Schoenberg, "Sommermorgen an einem See", Op. 16, No. 3
Paul Hindemith, "On hearing 'The Last Rose of Summer'"
George Crumb, Music for a Summer Evening (Makrokosmos III)
Fountains of Wayne, "It Must Be Summer"