Summer typically begins in two common ways:

1. Astronomical Summer: This starts on the summer solstice, which is the longest day of the year. In the Northern Hemisphere, the summer solstice occurs around June 20 or 21. It marks the point when the Earth's North Pole is tilted closest to the sun, resulting in maximum daylight hours. Astronomical summer lasts until the autumnal equinox around September 22 or 23.
2. Meteorological Summer: This is defined by the calendar months and seasonal temperature patterns. Meteorological summer always starts on June 1 and ends on August 31 in the Northern Hemisphere. It divides the year into four periods of three full months each for easier meteorological observation and comparison.

For 2025, astronomical summer in the Northern Hemisphere begins on Saturday, June 21, and ends around September 22.

In summary:

• Astronomical summer is from around June 20-21 to September 22-23.
• Meteorological summer is June 1 to August 31.

The same principles apply oppositely in the Southern Hemisphere, where summer starts around December 21-22.