One can have pumpkins and/or Halloween during the summer. Humans are the ones that decide when one season ends and the other begins. Just find a way to change the parameters.
Since seasons begin/end on the equinox or solstice, that is currently the parameter.
Summer still has another week to go. Winter doesn't start until most of the way through Dec. All the seasons' actual beginnings are about a month behind when it "feels like" that season started.