The evolution of a summer umbrella, a moniker for the stations’ activities during the Summer, pretty much began and ended with “The (Dial Position) Days Of Summer” in 1981. 99% of stations that have used it, didn’t use it so that it had any meaning. It was just a slogan.
Why should you have a Summer umbrella? To tie everything together. It’s a great way of owning a position. Like numbering your trip giveaways. It brands them for you. Do you need an umbrella? No. Of course not. But if you have something that you intend to focus your efforts on this Summer, then having something that gives continuity to it is great. We’re just not doing a zillion concerts haphazardly. It’s all part of (whatever).
BTW: I’ve always been a fan of freshening up the garments and designs. Back in the day, we used Club WLOL as our Summer logo. Like “Club Med” or another resort. Sporty little logo with, I believe, a parrot on it. Here are some, but not nearly all, the possibilities for umbrellas.