There are many discrepancies and ambiguities in the ideas of the researchers relating to the appearance and development of the lunar Hegira calendar in the Arabic peninsula during the first 18 years after the Emigration of H.H.Mohammad the Prophet. From the middle of the 2nd century of the Hejira era up to now, investigations have been carried out about the lunar Hejira calendar. Several tables and calculation methods have been provided for this purpose. However, they do not solve the problem satisfactorily. On this basis, I have prepared a computerized mathematical model for the subject which has led to two equations for the calculation of the conventional lunar Hegira calendar. These equation may be used for any year after the Hejira era and enable us to determine the leap years, the year-days and the week-days.
)