Islam is the world's second most followed religion.
Some Muslims disapprove of celebrating the birthday of the Prophet (pbuh), on the grounds that it is an innovation, and innovations in religious matters are forbidden.
Some Muslims say that if changes were made in religious matters it would imply that Islam was not complete when it was revealed to the Prophet Muhammad (pbuh), or that the Prophet (pbuh) did not tell Muslims everything that was revealed to him. This would be seen as highly sacrilegious by many Muslims.
Al-Hijra (1 Muharram)
This festival commemorates the Hijra (or Hegira) in 622 CE when the Prophet Muhammad (pbuh) moved from Mecca to Medina
Shi'a Muslims in particular use the day to commemorate the martyrdom of Hussein, a grandson of the Prophet (pbuh) in 680 CE.
Milad un Nabi(12 Rabi')
At this time Muslims think about Muhammad (pbuh), and the events of his life.
Lailat al Miraj(27 Rajab):
The festival celebrates Prophet Muhammad's (pbuh) journey from Mecca to Jerusalem in a single night on a strange winged creature called Buraq. From Jerusalem he ascended into heaven, where he met the earlier prophets, and eventually God.
Lailat al Qadr(27 Ramadan):
The festival of The Night of Power marks the night in which the Qur'an was first revealed to the Prophet Muhammad (pbuh) by Allah.
Ed ul Fitr (1 Shawwal)
This marks the end of Ramadan, the month of fasting, and is a festival of great celebration. In Islamic countries it is a public holiday.
Eid ul Adha(10 Dhul-Hijja)
This festival marks the end of the Hajj or holy pilgrimage, which is one of the 5 pillars of Islam. However it is celebrated by all Muslims, not just those who are on the pilgrimage.