To understand the Hijri calendar…we have to go back to the days of the Prophet Muhammed (pbuh). In 622 Common Era (CE) the Prophet Muhammed (pbuh) emigrated from Mecca to Medina (which was called Yathrib at the time). This was the point in time that was designated as the start of the Islamic Hijri calendar. The years are marked by the designation AH (after Hijrah). Each day in the Islamic calendar begins at sunset of the day before, and each month is calculated based on the lunar calendar. This means that each month is roughly 12 days shorter than the Gregorian calendar. Omar (ra) officially designated the Hijri calendar in 639 CE.
The emigration to Yathrib was a result of the difficulties that the Muslims faced in Mecca. The Muslims in Mecca were persecuted for their faith because of their opposition to idol worship and polytheism. Some Muslims found refuge in Ethiopia, then called Abyssinia, but not all were able to emigrate.
In the year of sorrow (am AL-Huzn), the prophet Muhammed’s (pbuh) wife Khaidja (ra) and his uncle Abu Talib (ra) died (619 CE) . His uncle, Abu Lahb, then assumed leadership of Mecaa and Beni Hashi. The opposition and persecution of the Muslims was unbearable.
During this period, Muhammed (pbuh) was invited to Yathrib by the tribal leaders to serve as their arbitrator and trusted advisor. They wanted his help to resolve issues that had existed between the tribes for over 100 years (especially after the battle of Bu’ath where all tribes suffered greatly).
Prior to this invitation, some of the people in Yathrib had already converted to Islam. They spoke highly of the Prophet (pbuh) and many others had a strong sense of trust in Muhammed (pbuh) for his past dealings with them in business and trade. In exchange for their vow to protect him as their own and to welcome the Muslims to Yathrib, Muhammed (pbuh) agreed to serve as their advisor.
At this, time there were many plots to kill the Prophet (pbuh) to thwart the Hijraa. He left with Abu Bakr (ra), and the Muslimeen of Mecca followed. The properties and wealth of the muhajreen (the emigrants) were seized by Meccans but this did not stop them from continuing their 200 mile journey to Yathrib.
After 8 days of travel the Muslims stopped at Quba, just outside of Yathrib, and Muhammed (pbuh) established the Mosque of Quba there. After 14 days, the Muslims arrived in the city of Yathrib, on a Friday and were able to join the Friday prayer. They were welcomed by the Ansar (the helpers) who helped make Yathrib a home and safe haven for the emigrating Muslims. Yathrib was then renamed to Medina to honor the prophet Muhammed (pbuh).
The Prophet (pbuh) drafted the Constitution of Medina which defined the rights and duties of all of the citizens of Medina. It protected each persons religious beliefs and cultural practices in exchange for political allegiance and a city tax. In this time, a sense of oneness, peace, and community was developed in Medina under Muhammed’s (pbuh) institution of Islamic governance.
An interesting fact is that during this period, there were many shared cultural practices between the people of the book (Muslims, Christians, and Jews). Muhammed (pbuh) allowed the Muslims to adapt the fasting of Ashurra or Yom Kippur (the day when the sea split for Moses (as) and the day when Noah (as) stepped off of the arch). Muslims differentiate themselves by fasting on a different day than the Jews. At this time, the Muslims also prayed toward Jerusalem (not Mecca as we do today, because it was still a center for idol worship). In this way, the Muslims, Jews, Christians, and Arabian Tribes were able to live together harmoniously in Median.