Nasi in Arabic was the original first month of the year in the Islamic calendar. On the tenth of Nasi they used to take a lamb out of the flocks to slaughter on the 14th (sunset on the 13th). This four day celebration was called Eid Qurbani, but if the spring equinox had not occurred before the 14th, Nasi was extended by an extra (intercalary) lunar month into Muharram to ensure the lamb would be slaughtered on the day before the first full moon after the equinox. After 632CE Nasi was never allowed to be spread over two lunar months again as Muharram, and the Muslim year was restricted to only ever 12 lunar months. For some time after this habit the first month was called Nasi by some and Muharram by others until eventually the 2nd Caliph Omar made the 12 lunar month system official and declared Muharram as the official name of the first month. At this point the month of Nasi became confined to the history books as Islam's lost "intercalary" month but the importance of 10th of the month before Muharram continued and developed into Eid ul-Adha.