The Islamic New Year takes place during the first month of the Hijrī, or Muslim lunar calendar. Though majority-Islamic countries are governed by the solar Gregorian calendar, the lunar calendar is used to calculate the dates of religious feasts and important observances such as the Hajj pilgrimage. Because the Hijrī relies on the movements of the moon, the Muslim calendar has just 354 or 355 days, making it about 11 days shorter than the solar Gregorian calendar with 365 days (366 in leap years).Umar I, the second Muslim caliph, instituted the calendar in 639 C.E. as part of a broader attempt to standardize and organize Islamic life and traditions—and possibly so the calendar would stand apart from those used by other religions.
Umar set the beginning of the calendar on an important anniversary: the summer in 622 C.E. during which the Prophet Muhammad and his followers are said to have migrated in secret from Mecca, the city in Saudi Arabia where the prophet was born, to Medina. That migration, also known as Hijrah, was an attempt to escape religious persecution, and the date is widely seen as the beginning of Islam both as an organized religion and political institution.
Though the Hijrī calendar has a distinct starting point, the days and times its months begin can vary by region because it relies on the first sighting of the new crescent moon.
Sourced from https://www.nationalgeographic.com/history/article/what-is-the-islamic-new-year-and-how-is-it-celebrated?loggedin=true&rnd=1684965529446
