Shab-e Yalda (“Yalda night” Persian: شب یلدا‎‎) or Shab-e Chelleh (“night of forty”, Persian: شب چله‎‎) is an Iranian festival celebrated on the “longest and darkest night of the year,” that is, in the night of the Northern Hemisphere’s winter solstice. Calendrically, this corresponds to the night of December 20/21 (±۱) in the Gregorian calendar, and to the night between the last day of the ninth month (Azar) and the first day [Khorram Rooz] of the tenth month (Dey)  of the Iranian civil calendar.

Khorram Rooz is the first day of Dey, the 10th month of the Iranian calendar. Both religious and non-religious traditions are associated with this day, such as the ancient Persians considering it the first day of their New Year.

This yalda night was the sixth anniversary of release Saaghar. New version is coming soon…

Wikipedia:

[1] https://en.wikipedia.org/wiki/Yaldā_Night

[2] https://en.wikipedia.org/wiki/Khorram_rooz