That is a tough question. Normally taglines are fairly easy to come up with, but for this Bradbury story, the best tagline available is the title of the story. I can picture the line "All summer in a day" at the bottom of a movie poster with some kind of sunrise appearing in the background.
A movie tagline should find a way to hint at the events of the story, while at the same time avoid spoilers. It should also really hook the reader's interest. That's why some movie taglines are in question format. It's an easy hook to your reader. "Are you in or out?" from Ocean's Eleven for example. Or "Who ya gonna call?" from Ghostbusters.
You could do the same thing. "What would you do if summer lasted only 1 day?"
You could go slang with the tagline. Grammar errors tend to stick in people's minds too. "This ain't no endless summer." That tagline has the added bonus of alluding to another famous movie -- Endless Summer