It can depend on the location but where I'm at it starts to get cooler towards night after the start of Fall which is September 22nd.
Then every night after that it begins to get cooler and cooler and when it gets to the middle of October the nights stay cooler and the days begin to cool off as well and by the end of October I usually need to wear a jacket during the day.
By November 1st I usually have to start turning on my heating and wear my heavier coat during the day and night.
The temps here usually start dropping to 50 F to 40 F for the highs during the day and 30 F to 40 F during the night during the month of November then the temperatures start falling even more after that until it warms up around May of the following year.
But of course the temperatures can vary from location to location since the weathers differs greatly in each State and part of the planet we're on.