The festival runs for three days. It opens on Kartik Shukla Chaturdashi, the fourteenth day of the bright fortnight of Kartik, which is also observed across Nepal as Baikuntha Chaturdashi. The second day falls on the Kartik full moon. The third and largest day is Mangsir Krishna Pratipada, the first day of the dark fortnight that opens the month of Mangsir.
In the Gregorian calendar that almost always lands in the second half of November. In some years the tithi slips far enough that the closing procession falls in the first days of December. There is no fixed Western date and anyone who gives you one has guessed. The day is set each year by the Nepali panchanga (the astronomical almanac) and confirmed locally by the temple guthi (the hereditary trust that funds and runs the festival). If you are planning a trip around it, ask the Mahalaxmi Temple committee or Chandragiri Municipality a few weeks ahead rather than trusting a date printed in English.
One practical note. Because the festival is anchored to Kartik Purnima, it sits about a month after Dashain and roughly a fortnight after Tihar. If you are already in the Kathmandu Valley for the autumn festival season, Thankot's jatra is the one that comes last, when the visitor crowds have thinned and the crowd on the street is overwhelmingly local.




