When is Vijaya Dashami?

The festival falls in Ashoj and Kartik (September and October), starting from Shukla paksha(bright lunar fortnight ) Ghatasthapana and ending on Kojagarat Purnima, the full moon. This festival is celebrated till 15 days.
 

Traditions of Vijaya Dashami 

Vijayadashami, also known as Dashain is the biggest and the longest festival of Nepal. This festival is observed for fifteen days. Ghatasthapana marks the beginning of the festival. Families worship Ashta-Matrikas (8 Tantrik Goddesses) and nine avatars of Durga, one on each day. On the tenth day, families and relatives gather and put Tika and receive blessings from the elders. This usually spread over a span of 5 days. This festival also cherishes the power of femininity and annihilation of evil. The last day of the festival falls on full moon day and is called Kojagrat Purnima.

The festival represents the victory of Lord Durga over the Demon 'Mahishasur'. The festival as a whole is the celebration of victory of virtuous over evil.