Muhammad's first revelation was an event described in Islamic tradition as taking place in AD 610, during which the Islamic prophet Muhammad was visited by the angel Gabriel, who revealed to him the beginnings of what would later become the Qur'an. The event took place in a cave called Hira, located on the mountain Jabal an-Nour near Mecca. It is not mentioned anywhere when was the exact date and time of the revelation but according to Mubarakpuri, the exact date of this event was Monday, the 21st of Ramadan just before sunrise, i.e. August 10, 610 C.E. – when Muhammad was 40 lunar years, 6 months and 12 days of age, i.e. 39 solar years, 3 months and 22 days. Date of the revelation The state of the calendar at the time of the first revelation Muhammad was born 55 days after the incident of the elephant, which occurred in the middle of Muharram 570 - i.e. his birth date was Monday, 12 Rabi'I in that year. Many military campaigns occurred around the turn of the year, when conditions were propitious for fighting. Additionally, trade, and gatherings associated therewith, was largely seasonally based. To stop the calendar months from rotating through the seasons, intercalation was employed. This involved the occasional insertion of an extra month (announced at the pilgrimage), ideally seven times in nineteen years. Intercalation was said to have been introduced in AD 412, and it was borrowed from the Jews. The Jewish official controlling the practice was known as Nasīʾ. When the Arabs adopted the procedure they used this word nasīʾ to denote the whole system. It was operated similarly to the way the Jews operated it - the beginning of the year (Muharram) was tied to the spring season. Identifying the date of the first revelation One can establish the day of the Islamic month (but not the month itself) corresponding to a given Julian date by projecting the fixed (i.e. non-intercalated) calendar backwards. When they do, they will find that 6 August 610 corresponds to 11 Ramadan and 10 August to 15 Ramadan. The first revelation is not looked for earlier than the last ten nights of the month. The equivalence of 40 lunar years, 6 months and 12 days to 39 Gregorian calendar years, 3 months and 22 days is also incorrect. 39 Julian calendar years, 3 months and 22 days takes us back from