Had the opposite situation – our ad agency got sick of trying to get hold of clients on a friday. During the summer, you could call the manager, your main contact, the VP, the CEO… and just be leaving urgent messages. So we went to summer hours.
btw, Hope you left voluntarily, kenobi… I did, and life after is much less stressful.
This is somewhat what I thought the OP would be about, but my idea was more those “industries” that are open seasonally during the summer: beaches (and the ancillary services, i.e., boardwalk businesses,etc), tiny roadside ice cream places, roadside produce stands, etc.