It's not actually the longest day...there are still 24 hours in the day on June 20th. However, because it's the summer solstice, we WILL have the greatest amount of daylight that we'll see all year at over 14 hours and 18 minutes.
With a mostly clear sky, all that sunshine will make for another hot one. We won't have much of a breeze, so you'll really be feeling that afternoon high near 90°. The humidity will gradually starting creeping upward through the weekend, too. A stray shower isn't totally impossible Saturday, but better rain chances are in the forecast Sunday.
For Father's Day on Sunday, it will be equally as hot as Saturday with a high in the lower 90s and morning temperatures in the upper 60s. Expect a few showers and thunderstorms to develop as well, but any outdoor plans shouldn't be a washout. The pattern gets wetter next week with an approaching cold front.
Scattered showers and storms develop by Monday afternoon, becoming more widespread Tuesday through Wednesday. The cold front is a slow moving one, basically stalling out over North Alabama by midweek. Temperatures will at least be a bit lower, in the lower 80s Wednesday and Thursday. In total, most locations will pick up between one and two inches of rain next week.