Usage patterns
秋になるfor autumn to arrive; to become autumn
It'll be autumn soon, won't it?
秋が深まるfor autumn to deepen (get further into the season)
Autumn deepened, and the leaves on the trees began to turn color.
〈事〉の秋autumn as the season best suited for (doing) something
As they say, autumn is the season for good appetite—I've been eating a lot lately.
秋に入るto move into autumn (as a season)
September has come, and by the calendar we've entered autumn.
