For Muslims all over the world, Wednesday night marks Hijra, the first day of the month of Muharram and the beginning of the Islamic New Year. Hijra means "migration" in Arabic, and it marks the beginning of the historic journey the Prophet Muhammad, the founder of Islam, made from Mecca to Medina in the year 622.

Centuries later, Hijra generally passes with little fanfare, especially compared with holidays like Eid al-Adha or Eid al-Fitr, even though Hijra is a public holiday in some countries. There are no particular rituals or traditions to celebrate it, although some Muslims will use the opportunity to make resolutions for the coming year, according to the BBC. 

But Hijra is nonetheless important because it signifies the birth of a community, centuries ago, that today includes more than 1.5 billion people. According to Islamic history, Muhammad, born in 570 in Mecca, was meditating one night in the year 610 in a cave on a mountain when the angel Gabriel came to him and told him to recite. Those recitations, which Muhammad believed were words from God, formed the basis of the Quran, the sacred Islamic scripture. 

Muhammad began to preach what he had heard, gathering a local following in the process. But as his popularity increased, so did the perception that he was a threat. In 622, Muhammad and his followers left Mecca for Medina, a city more than 200 miles away. There, he gathered enough adherents to eventually return to and take Mecca.

Hijra also marks the beginning of the month of Muharram, the first month of the Islamic calendar and one of four holy months. For Shiite Muslims, the first 10 days of Muharram are days of mourning for the Imam Hussain, the grandson of Muhammad, who was killed in a battle in 680.

Because the Islamic calendar is lunar, it changes every year with respect to the Gregorian calendar, which is solar. Hijra was celebrated last year Oct. 25; this year it begins at sunset Wednesday. Next year, Hijra will shift another 11 days earlier.