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Astronomical seasons begin at the equinoxes and solstices, which fall around 20 March, 21 June, 22 September and 21 December. The hemispheres are offset by six months, so the March equinox opens spring in the north and autumn in the south. Meteorological seasons instead start on the first of March, June, September and December.
Astronomical seasons
begin at the equinoxes and solstices
Hemisphere offset
southern seasons are shifted by six months
Because the tropical year is 365.2422 days, so each event drifts about six hours later annually until a leap day resets it.
Meteorological seasons for climate statistics, since they align with calendar months. Astronomical seasons for daylight and solar geometry.