Who decided when the seasons would officially start?

The official start of the seasons is largely determined by the astronomical calendar, which is based on the Earth’s position in relation to the sun. Traditionally, the vernal equinox marks the start of spring, the summer solstice begins summer, the autumnal equinox signals the start of fall, and the winter solstice marks winter's onset. These definitions have been established through centuries of observation and have been formalized in various calendars, including the Gregorian calendar. Different cultures may also have their own seasonal definitions based on local climate and agricultural practices.