The last day of summer depends on the definition of summer used:

• Astronomical summer in 2025 ends on Monday, September 22, 2025. This is marked by the autumnal equinox, when day and night are nearly equal in length. It is considered the official end of summer in the Northern Hemisphere.

• Meteorological summer ends on August 31, 2025. This definition splits the year into four equal parts of three months each, so summer is June 1 to August 31.

• In the U.S., Labor Day on September 1, 2025, is often regarded as the unofficial last day of summer for practical purposes, marking a traditional end to summer activities.

In summary, the exact last day of summer can vary from August 31 (meteorological summer) to September 22 (astronomical summer) in 2025. Labor Day on September 1 is commonly seen as the informal end for many in the U.S..