When Does Summer Official Start 2025. In 2025, the summer solstice will occur on june 20, at 20:51 gmt in the northern hemisphere, and on december 21, at 09:20 gmt in the southern hemisphere. The longest day of summer is the summer solstice, which.

The official start to astronomical summer in the northern hemisphere occurs this year on thursday, june 20, at 3:50 p.m. Here’s everything you need to know about the northern.