Durin's Day is a rare event noted by Dwarves.
The new year of their calendar was the last cycle of the Moon to begin in autumn, or in Thorin's words "the first day of the last moon of Autumn on the threshold of Winter". When on this day both the Sun and Moon may be seen in the sky together, it is called Durin's Day.
Before the History some calculations were made. According to Andreas Möhn, the Durin's Day of T.A. 2941 occurred on 22 October of the Shire Calendar. This is based in the assumption that the orbital period of the moon, as described in The Lord of the Rings, was about 20 minutes shorter/faster than today.
- J.R.R. Tolkien, The Hobbit, "A Short Rest"
- J.R.R. Tolkien, John D. Rateliff (ed.), The History of The Hobbit, Return to Bag-End, "The Fifth Phase", "Timelines and Itinerary", "iii. The Timeline Revisited", pp. 826,828
- Andreas Möhn, "The Moon and Durin's Day", lalaithmesp.blogspot.com (accessed 15 February 2018)
- Karen Wynn Fonstad (1991), The Atlas of Middle-earth, "The Hobbit", p. 100