Official Last Day Of Summer 2024. Is it really the longest day of the year? The last day of summer is determined by the september or autumn equinox, which marks the first day of fall.
You can check the spring, fall and winter dates for the years 2024, 2025 and 2026 on our four seasons. Thursday, june 20 is the official first day of northern astronomical summer for 2024 โ summer solstice โ which is the longest day of the year for this half of the planet.