What Is The Official First Day Of Summer 2024.  — summer officially arrives in the northern hemisphere on thursday, the earliest start to the season in over two centuries.  — the first official day of summer astronomically for 2024 is thursday, june 20, which is when the northern hemisphere has the longest day of the year, according.

I n 2024, june 20 marks the summer solstice, the first official day of astronomical summer.  — in the northern hemisphere this year, the first day of summer is thursday, june 20, 2024.