Al-Hijra / Muharram in 2012 will start on Thursday, the 5th of November and will continue for 29 days until Thursday, the 13th of December.
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 Wednesday, the 4th of November.
Although Al-Hijra / Muharram is always on the same day of the Islamic calendar, the date on the Gregorian calendar varies from year to year, since the Gregorian calendar is a solar calendar and the Islamic calendar is a lunar calendar. This difference means Al-Hijra / Muharram moves in the Gregorian calendar approximately 11 days every year. The date of Al-Hijra / Muharram may also vary from country to country depending on whether the moon has been sighted or not.