Hijri

The Islamic, Muslim, or Hijri calendar (Arabic: التقويم الهجري‎ at-taqwīm al-hijrī) is a lunar calendar consisting of 12 months in a year of 354 or 355 days. It is used in Muslim countries to determine the proper days of Islamic holidays and rituals, such as the annual period of fasting and the proper time for the pilgrimage to Mecca.

5 Questions

I have heard that Umar Ibn Khattab introduced Muharram as the first month of the Islamic calendar whereas earlier Rabiul Awwal used to be the first month. Is this true? Sayyed Mohammad Al-Musawi	How are the calendar dates calculated for events that happened before Prophet Mohammad PBUH such as the events related to previous Prophets? Amina Inloes
Which calendar, if any, was in use by the Muslims during the lifetime of the Prophet (saw)? Amina Inloes	If I look at an Islamic calendar, and it says Tuesday is a certain Islamic date, does that date start on Tuesday sunset time or on Monday? Amina Inloes