I should have posted this last week, on the day before the solstice, but timing isn't everything. Of course, to a great many here, hibernal (of, pertaining to, or taking place in winter) might seem more appropriate, but I like to look on the sunny, if not summer, side of life. Though of course, locally we tend to (a)estivate, to sleep through the summer (instead of hibernate, through the winter). Anyway, the word at hand was borrowed in the 14th century from Latin aestīvālis, from aestās, summer.
Now that the aestival solstice has come, the weather should start cooling down in a month or three.