Dashami is the tenth tithi of the bright fortnight, a Purna or full tithi in the fivefold grouping and so read as stable and favourable. Its presiding deity is traditionally Yama, lord of dharma and death, giving the day a note of righteous order. Its best-known festival is Vijayadashami, also called Dussehra, kept on the tenth tithi of the bright fortnight of Ashvin: it celebrates the victory of good over evil, marking both Rama's defeat of Ravana and, in the Goddess tradition, Durga's slaying of the buffalo-demon. The waxing Moon by now is broadly gibbous. In the panchanga Dashami is a day for successful completion and auspicious action. These are the conventions of the calendar and its festivals.
Vijayadashami's placement on the tenth tithi immediately after the nine nights of Navaratri makes the pairing of the two observances structurally significant: the goddess is worshipped for nine tithis corresponding to her nine forms, the Navadurga, and the victory is proclaimed on the tenth, so the festival calendar itself enacts the numerical logic of the tithi sequence. Yama's association with the tenth tithi as lord of dharma rather than purely of death reflects the wider Puranic understanding of Yama as an upholder of cosmic and moral order, a role distinct from, though related to, his more familiar function as the god who receives the dead, and this dharmic emphasis is part of why the tenth tithi carries a note of righteous triumph rather than of mourning.