The festival of lights — the Amavasya night when Lakshmi visits homes lit with lamps.
Diwali marks the new-moon night of Kartika month, the darkest night of the lunar year. Lighting lamps symbolises inner illumination and the welcome of Lakshmi, goddess of prosperity. In the Ramayana tradition it commemorates Rama's return to Ayodhya after 14 years of exile; in the Mahabharata tradition, the Pandavas' return after their exile.
Observed when Amavasya tithi is present during Pradosha Kala (the 2h24m window after sunset). The pradosha rule means Diwali's date can shift by one day across cities depending on local sunset, especially in years where Amavasya straddles the sunset boundary.
Computed for Chalk River (America/Toronto) using local sunrise, sunset and astronomical positions.