The date on which the fall season starts differs depending on the Hemisphere you are talking about. Astronomically, the first day of autumn is on the 22nd of September in the Northern Hemisphere. On the 22nd of November, autumn is over in the Northern Hemisphere but it is only an astronomical conclusion that differs slightly from country to country. As one can probably imagine, it is quite the opposite case in the Southern Hemisphere, where the fall season starts on the 22nd of March and ends on the 22nd of June. These estimations too, are astronomical and thus are not exactly the same for all countries within the Southern Hemisphere.
An Equinox is a date on which both the day and the night have equal number of hours. The Fall Equinox or the Autumnal Equinox is the date on which the sun is perpendicular to the Equator and also the date which marks the beginning of autumn. However, in reality, it is not possible to predict the exact day on which autumn or any other season would begin. Meteorologists have simplified the term of the seasons by assuming that each season starts on the first day and ends on the last day of a month and also that it lasts for three months each. Therefore, an answer to the question as to when fall season starts according to meteorologists is 1st September.
Apart from the astronomical and meteorological dates, there are other much simpler, yet more practical ways to tell when fall starts. Autumn is the season that comes just before winter and before the leaves fall out completely in winter, they start to change their color during the fall. Although it is in winter that the trees lose most, if not all of their leaves, the falling of the leaves starts in this season and has been a prominent sign from the ancient times. Another sign of the beginning of fall include the arrival of the harvest season with long nights, indicative of the cold and long winter nights that are about to follow.