Origin of the autumnal ceremony Sharadiya
The actual worship of the Goddess Durga as stipulated by the Hindu scriptures falls in the month of Chaitra, which roughly overlaps with March or April and is called Basanti Durga Puja. This ceremony is not observed by many and is restricted to a handful in the state of West Bengal. The more popular form, which is also known as Sharadiya Autumnal Durga Puja, is celebrated later in the year with the dates falling either in September or October. Since the Goddess is invoked at the wrong time, it is called Akaal Bodhon in Bengali.One of the earliest evidence of the autumnal celebration can be found in the Tripura Buranji, where it is described that the envoy of the Twipra Kingdom, Rameshwar Nyayalankar, was invited to witness the Durga puja at Rangpur, the capital of the Ahom kingdom, in 1711.
There is also a mythological belief that Lord Ram, who was a Durga worshiper, worshiped Goddess Durga before going for a war with Ravana. Rama had performed Chandi Homa and invoked the blessings of Durga, who blessed Rama with secret knowledge of the way to kill Ravana. On the day of Ashvin Shukla Dashami, Ramas party found Sita and defeated Ravana. This day is thus also celebrated as Dussehra.