There are two official holidays in Islam, Eid al-Fitr and Eid al-Adha, which are celebrated by Muslims worldwide. Both holidays occur on dates in the lunar Islamic calendar, which is different from the solar-based Gregorian calendar, so they are observed on different Gregorian dates every year. There are a number of other days of note and festivals, some common to all Muslims, others specific to Shia Islam as a whole or branches thereof. Both Eid al-Fitr and Eid al-Adha follow a period of 10 holy days or nights: the last 10 nights of Ramadan (Eid al-Fitr), and the first 10 days of Dhu al-Hijjah (Eid al-Adha). The Night of Power, one of the last 10 nights of Ramadan, is the holiest night of the year. Conversely, the Day of Arafah, the day before Eid al-Adha, is the holiest day of the Islamic year. Additionally, Friday is considered the holiest day of the week, and in Islamic tradition, is considered a celebration in itself. Friday Prayers (Juma) are congregational prayers held in mosques, and Muslims are encouraged to wear clean and refined clothes, perfume, and bathe. It's customary to eat special meals with family on this day. Eid al-Fitr and Eid al-AdhaEid al-Fitr is celebrated at the end of Ramadan (a month of fasting during daylight hours), and Muslims may perform acts of zakat (charity) on the occasion, which begins after the new moon is sighted for the beginning of the month of Shawwal. Celebration begins with prayers on the morning of 1 Shawwal, followed by breakfast, and often celebratory meals throughout the day. Eid al-Adha is celebrated on the tenth day of Dhu al-Hijjah, when the Hajj pilgrimage takes place and lasts for four days. Muslims may perform an act of zakat and friendship by slaughtering a sheep and distributing the meat to family, friends, and to the poor. Muslims are also encouraged to be especially friendly and reach out to one another during this period. Ramadan Muslims celebrate when the Quran was first revealed to Prophet Muhammed by fasting from dawn to sunset during Ramadan, the ninth month of the Islamic calendar.