Muḥarram, which is the first month of the Islamic calendar and considered the second holiest month after Ramadan, is slated to commence from Tuesday.
The Islamic or Hijri calendar, consists of 354 or 355 days, making it 11 days shorter than the Gregorian calendar. This year will be called Hijri 1443 AH (Anno Hegirae in Latin). It means that it has been 1443 years since Prophet Muhammad was forced to migrate to Medina from Mecca. The Islamic calendar ends with the month of Dhul al-Hijjah, which means the month people go for Hajj to Mecca.
Since the calendar is based on sightings of the moon, several authorities will announce the commencement of the Islamic new year based on when the crescent moon is sighted. The moon-sighting committee of the Kingdom of Saudi Arabia said 10 August will be the first day of Muharram.
History of Islamic New Year:
The Islamic calendar began in 622 AD when the Prophet and his companions were forced to leave Mecca. They were forced to move to Medina to avoid religious persecution. The Prophet later returned to the city after he conquered it in 629 AD.
Significance of Islamic New Year:
Although the first day of the month is of immense importance, there are no large-scale celebrations held in Muslim-majority countries to commemorate it. Many countries like Saud Arabia and the United Arab Emirates mark the occasion with a public holiday.
The first 10 days of the month of Muharram are of great importance to Shia Muslims in particular. They observe it as a solemn period. Imam Hussein, the grandson of Prophet Muhammad was martyred on the tenth day of Muharram in the Battle of Karbala in 680 AD. The day, known as Ashura, is marked as a day of mourning by Shias.
source https://www.firstpost.com/india/islamic-new-year-2021-date-significance-of-second-holiest-month-in-islam-9876711.html
