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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.
In Persian, "Khorram Rooz" translates to "happy day". The Muslim scholar Abū Rayḥān al-Bīrūnī wrote that the month Dey is also called khor. The first day of the month is called khorerooz. The Zoroastrians call it dey dadar jashn.
The followers of the Mithraic mysteries consider the first day of Dey to be the birthday of the Sun, because the days lengthen afterwards. Ancient Persians used to start their years with the first day of winter and the first day of the sun, making khorram rooz their Nowrooz—the first day of their year.
- Razi, Hashem, Calendars and the ceremonies of ancient Iran, Tehran, Faravahr publisher, 1358