Estival means "summery." When a sunny day in April has an estival feel to it, it's one of those days that seems so much like summer that you might be tempted to go barefoot.
The adjective estival is rarely used, and it's usually found in formal or technical writing. However, there's no reason not to use it when you want to describe something that has to do with summer, like a blue estival sky. Estival comes from the Latin word aestus, or "heat." There are equivalent adjectives for the other seasons, too — autumnal for autumn, hibernal for winter, and vernal for spring.