The simple answer to when do the seasons start depends on which system you use. Two systems define the seasons. Astronomical seasons begin at the equinoxes and solstices. Meteorological seasons start on fixed dates. Knowing the difference tells you which dates to mark on your planner, whether you are tracking the first day of fall or the first day of spring.
Astronomical Seasons Based on Earth’s Orbit
Astronomical seasons are tied to the position of Earth in its orbit around the Sun. They begin at the equinox and solstice dates. In the Northern Hemisphere, four events divide the year:
- Spring starts at the March equinox, around March 20.
- Summer starts at the June solstice, around June 20 or 21.
- Autumn starts at the September equinox, around September 22 or 23.
- Winter starts at the December solstice, around December 21 or 22.
Earth’s orbit is not a perfect circle. The exact date and time of each event can shift by a day or two each year. This system has been used for centuries and aligns with the changing length of daylight.
Meteorological Seasons Based on Calendar Months
Meteorologists use a simpler system: meteorological seasons begin on the first day of a month. Whole months make climate statistics easier to compare across years. In the Northern Hemisphere, the seasons are:
- Spring: March 1
- Summer: June 1
- Autumn: September 1
- Winter: December 1
Every March is fully in spring, so temperature and precipitation records line up neatly. Weather services and long-range planners rely on this system.
Which System Should You Use for Your Planner?
Your choice depends on what you are planning. Track natural events like the longest day of the year or the day of equal day and night? Mark the equinox and solstice dates. These are the dates tied to the first day of fall or the first day of spring in many cultures.
Organising monthly schedules, weather records, or school terms? The meteorological system is more practical. Fixed dates let you plan ahead without checking the exact astronomical moment each year.
Marking Season Start Dates
Add season start dates to a monthly planner in a few steps:
- Decide which system you want to follow: astronomical or meteorological.
- For astronomical seasons, look up the equinox and solstice dates for the current year. They shift slightly.
- For meteorological seasons, write the first day of the month: March 1, June 1, September 1, December 1.
- Write the season name next to the date.
- Optional: note the season change in both systems to compare them.
Either way, you get a clear reference for the start of each season.
Southern Hemisphere Reversal
In the Southern Hemisphere the seasons are reversed. Spring in the north means autumn in the south. The astronomical dates stay the same, but the season names swap. The March equinox marks the start of autumn south of the equator, not spring. The same applies to meteorological seasons: March 1 is the first day of autumn.
Why Two Systems Exist
The two systems serve different purposes. Astronomical seasons follow the natural cycle of sunlight and Earth’s tilt. They are linked to the solstices, the longest and shortest days, and the equinoxes, when day and night are equal. Meteorological seasons are a human convention that groups months into blocks of three for calculating averages and trends in temperature and precipitation. Neither system is more correct. They are different tools for different tasks.
Practical Tips for Planning
Printing a monthly planner from septembercalendar.info? Note the season start dates in the margin or at the top of the month. For the meteorological system, add a note on the first day of March, June, September, and December. For the astronomical system, check the current year’s equinox and solstice dates. They vary slightly. Search for “equinox dates” to get the exact times. Write them on the planner as a reminder of the first day of spring, the first day of fall, and the other season starts.