New Islamic calendar year, and is the day on which the year count is incremented. The first day of the year is observed on the first day of Muharram, the first month in the Islamic calendar. The first Islamic year beginning in 622 AD during which the emigration of Muhammad from Mecca to Medina, known as the Hijra.
While some Islamic organisations prefer determining the new month (and hence the new year) by local sightings of the moon, most Islamic institutions and countries, including Saudi Arabia, follow astronomical calculations to determine future dates of the Islamic calendar.
There are various schema for calculating the tabular Islamic calendar (i.e. not based on observation), which results in differences of typically one or even two days between countries using such schema and those that use lunar sightings. For example, the The Umm al-Qura Calendar used in Saudi Arabia was reformed several times in recent years. The current scheme has been introduced in AH 1423 (15 March 2002).
A day in the Islamic calendar is defined as beginning at sunset. For example, 1 Muharram 1432 was defined to correspond to 7 or 8 December 2010 in official calendars (depending on the country). For an observation-based calendar, a sighting of the New Moon at sunset of 6 December would mean that 1 Muharram lasted from the moment of sunset of 6 December to the moment of sunset of 7 December, while in places where the New Moon was not sighted on 6 December, 1 Muharram would last from the moment of sunset of 7 December to the moment of sunset of 8 December.