Autumn, interchangeably known as fall in the US and Canada, is one of the four temperate seasons. Autumn starts at the beginning of the school year. 
Autumn in poetry has often been associated with melancholy. The possibilities of summer are gone, and the chill of winter is on the horizon. Skies turn grey, and many people turn inward.
 Autumn is associated with the Halloween season (a Celtic autumn festival).
So with the beginning of autumn, we'll be happy because of  colors, weather and holidays...