What happens: The summer solstice is the longest day of the year. The North Pole is tilted maximally toward the sun. While many consider this "mid-summer," astronomically, it marks the beginning of summer. Day length slowly decreases after this point.
The answer depends entirely on your context.
What happens: The winter solstice has the fewest hours of daylight. The North Pole is tilted farthest from the sun. Ironically, this is when the "cold season" officially begins astronomically, even though many locations have already experienced freezing weather.
Why the dates shift: The astronomical dates vary because a tropical year (Earth’s orbit) is not exactly 365 days—it is roughly 365.2422 days. This is why we have leap years, which push the solstice and equinox dates by about six hours each year, causing them to land on different calendar days.
Every year, as the air warms, the leaves turn, or the first snowflake falls, we casually refer to the "start of a new season." But if you ask five different people when winter officially begins, you might get five different answers. Does winter start with the winter solstice (around December 21), or on December 1? Is the first day of summer June 1 or June 20?
The confusion is understandable. There isn't just one universal answer to the question, "When do the four seasons start and end?" In fact, scientists, meteorologists, and astronomers use two very different systems to define the seasons.
This article breaks down both methods—astronomical seasons (based on Earth’s orbit around the sun) and meteorological seasons (based on the annual temperature cycle). By the end, you will understand exactly when each season begins and ends, why the dates shift, and which system is more useful for your daily life.
The astronomical definition is likely the one you learned in school. It is based on the Earth’s 23.5-degree axial tilt and its 365-day journey around the sun. Seasons change when the Earth reaches four key points in its orbit: the two solstices and the two equinoxes.