Ramadan is based on the lunar calendar and begins with the hilal, the Arabic word for crescent or "new moon," in the ninth month of each year. But because the lunar cycle steadily moves backward compared to the Gregorian calendar, Ramadan falls earlier and earlier each year. During the summer months, the days are much longer and hotter, making fasting more difficult. Late in the holy month, Muslims celebrate the Laylat al-Qadr, or Night of Power. This is the anniversary of the night on which Allah first revealed the Koran to the Prophet Muhammad. While different traditions celebrate this night in the last 10 days of Ramadan, the 27th night is the most widely celebrated date, when the Koran says, "The grand night is better than a thousand months."