The first day of summer changes annually, arriving on June 20, 21 or 22. That's because the earth's astronomical year is actually 365.25 days long, writes Space.com.

The first day of summer changes annually, arriving on June 20, 21 or 22. That's because the earth's astronomical year is actually 365.25 days long, writes Space.com.

Today will be the longest day of 2025 in the Northern Hemisphere, kicking off the coveted summer season. The 2025 summer solstice is expected to officially start on June 20 at 10:42 p.m. ET ...