The Islamic New Year is the day that marks the beginning of a 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.
Since the Islamic year is 11 to 12 days shorter than the Gregorian year, the Islamic new year does not come on the same day of the Gregorian calendar every year. While some Islamic organizations prefer determining the new month (and hence the new year) dates by local sighting of the moon, most Islamic institutions and countries, including Saudi Arabia, follow astronomical calculations to determine future dates of the Islamic calendar. The following dates on the Gregorian calendar correspond to the Islamic new year.
Al-Hijra / Muharram in 2010 will start on Tuesday, the 7th of December and will continue for 29 days until Tuesday, the 4th of January.
Note that in the Muslim calander, a holiday begins on the sunset of the previous day, so observing Muslims will celebrate Al-Hijra / Muharram on the sunset of Monday, the 6th of December.