What is Diwali, and how is its date fixed?

A light held up for those who came before

The verse above is not about wealth. "When the Sun is in Tulā, at the evening of the Chaturdaśī and the Amāvāsyā, let people take a torch in their hands and show the ancestors their way." Dharmasindhu gives the mantra that goes with it: may those of our family who have passed on see the path by this bright light, and go on.

That is one root of the row of lamps: dīpa-dāna, the giving of light, at the doorway, the temple, the garden, the cattle shed, on each of the three nights from Chaturdaśī to Pratipadā.

Lakshmi at pradoṣa

The same text sets out the evening of the Amāvāsyā in order: an oil bath in the morning, worship of the household deities, the monthly offering to the ancestors in the afternoon, and at pradoṣa the lamps, the torch, and the worship of Lakshmi. Only then the meal. Children and the old are excused from waiting.

After midnight, it adds, the women of the town sweep Alakshmī, want and neglect, out of the courtyard. A clean house is the oldest part of the festival.

How the date is decided, and why some years show two

A tithi doesn't follow the clock. The Amāvāsyā can begin in the middle of one day and end in the middle of the next. Dharmasindhu's rule is about the evening: if the Amāvāsyā covers pradoṣa on the first day only, Lakshmi puja is that day; if it covers both evenings, or only the second, it is the second.

In 2026 the Amāvāsyā begins at 11:28 am on 8 November and ends at 12:32 pm on 9 November, so only the evening of the 8th has it. When two calendars disagree, it is almost always because the Amāvāsyā touches both evenings at their city and they read the rule differently. We decide the day once, at Ujjain, and show the local window for your city. The day is the same everywhere; the times are your city's.

Kārtika or Āśvina?

Both. Calendars that end the month at the full moon (most of north India) call this new moon Kārtika's. Calendars that end the month at the new moon (Maharashtra, Gujarat, the south) call it the last night of Āśvina. Dharmasindhu, written in the second system, files Diwali under Āśvina. It is one night.

The muhurat, and what it rests on

The tradition sets Lakshmi puja at pradosha: the first three muhurtas of the night after sunset (Nirnaya Sindhu). Within pradosha, panchangs by long custom choose the time when the fixed sign Vrishabha is rising, the same fixed-sign principle the texts give for installing a deity, so that what is welcomed stays. The table above gives both windows for your city, computed for this year. We don't tell anyone that a time is lucky or unlucky for them, and a puja done with care at any time in pradosha is within the tradition.

Where this comes from

The rules for the date and the order of the evening are from Dharmasindhu (Kāśīnātha Upādhyāya, c. 1790), checked against Nirṇaya Sindhu. The tithi times and the city windows are computed with Swiss Ephemeris. Household practice varies from family to family and is recorded here as custom, not rule.