We have seasons because Earth has a wobble as it turns. Sometimes it wobbles so the north half of the planet gets most of the light, and sometimes it wobbles so the southern half gets most of the light. On March 21 (usually) and September 21 (usually) the days are of equal length. In the northern half of the world, after March 21 the Earth slowly tilts the North Pole towards the sun until June 21 (usually) then starts tilting back the other way. The days are longer than the nights during this time. After September 21 the Earth tilts the South Pole towards the sun. After December 21, it starts tilting back the other way. The days are shorter than the nights during this time.
Do you see where this is going? March 21 is the first day of Spring, June 21 the first day of Summer, September 21 the first day of Autumn, and December 21 the first day of Winter.