Dashanga Lalitha Vratham is observed for ten days in Ashwin Month. In 2017, Dashanga Lalitha Vratham begins on September 30 and ends on October 9.
It is observed from Shukla Paksha Dashami to Krishna Paksha Chaturti in Ashwin Month.
During this period Goddess Lalitha and Rohini Devi, consort of Chandra deva are worshipped with ten flowers, deepam, offered ten fruits. Ten Suvasini (the married women) are worshipped. Ten namaskaras will be done.
Every night after chandrodaya (moonrise) the performer of the vrata should offer ten ksheera arghyas in shanka (cow milk in a conch) to Chandra and Rohini.
According to Bhavishyottara Purana, King Dasharatha performed this vratha under the guidance of Rishyashringa Maharshi in order to get Vishnu as his son.