aye ...This is what I thought. Around 21st march and 21st September are usually the equinoxes signalling the official start of spring and autumn and 21st June and December for the solstices signalling the start of summer and winter.
This year the autumn equinox in the northern hemisphere takes place on September 23 - the same day as the beginning of astronomical autumn.
The autumn equinox marks the day when the northern hemisphere slightly tilts away from the sun, meaning days become shorter and the temperature begins to cool.
This carries on until the winter solstice