Prophet Muhammad's Birthday is celebrated during the month of Rabiulawal, the third month of the Muslim calendar. Since the calendar is based off the moon there is no certain date that it occurs on and it honors the birthday of the Prophet Mohammed.
This purely religious holiday is celebrated with the reciting of scripture from the Koran as well as some religious lectures. It is a public holiday in Egypt that most people get off.
The base of the holiday starts with the fact that Prophet Muhammad was born on 12 Rabiulawal in 570 AD. Early celebrations of this day were seen in 8th century Mecca when the people turned the place where he was born into a place of worship.