Summer was a season.
The Summer of 1967 was known as "the summer of love." (DS9: "Past Tense, Part II")
Temperatures of 28° below zero were not uncommon in the middle of summer in the Northern Wastes of Andoria. (ENT: "The Aenar")
Tom Paris wore a hat to cover the haircuts his father Owen Paris forced him to get every summer. (VOY: "Faces")
After the USS Defiant was sent back in time to the 22nd century and crashed on the planet Gaia, Jadzia Dax cut her hair short during the summer after the crash. (DS9: "Children of Time")
In mid-2374, Worf suggested to Jadzia that they should hike across Vulcan's Forge for their honeymoon. Jadzia was not keen to honeymoon there at all - especially in summer. (DS9: "Change of Heart")