Which preposition to use with summertime

for Occurrences 1%

I lived there from the time I was ten years old until I was twenty-three, and always go there in summertime for a few weeks' restoccupying my old room, eating with the boys, and roaming in the woods; I know every tree and bypath; yes, and many a swim have I had in the old river.

Which preposition to use with  summertime