The Islamic calendar, the Hegira, is based on the lunar cycle. Likewise the Julian calendar the Hegira too has 12 months in each year. Every month begins with the new moon. Thus the Islamic year falls short of the international Gregorian one by 11 days.
Based on this lunar year, the festivals of the Muslims travel through each season at every few years interval. The cycle of years is divided into segments of 30 years. Of these, 19 years have 354 days each and the next 11 years have an extra day each.
The years of the Muslim calendar are lunar and always consist of 12 lunar months alternately 30 and 29 days long, beginning with the approximate New Moon. The year has 354 days, but the last month Dhu al-Hijjah sometimes has an intercalated day, bringing it up to 30 days and making a total of 355 days for that year. The months do not keep to the same seasons in relation to the Sun, because there are no intercalations of months. The months regress through all the seasons every 32 1/2 years.
The day of Jumma:
The Islamic week has 7 days of which Jumma or Friday, being the day of rest, is the most important and sacred. Hence, it is also regarded as the day of assembly. This tradition was instituted by Prophet Muhammad who claimed it to be a divine command. According to Islamic belief, Friday is the best day on which the sun rises. It is also the day on which Adam, the first man, was taken into Paradise and turned out of it. The day on which he repented and on which he died. And Islam believes, it will also be the day of judgement. There is also a certain hour on Friday, known only to god, on which a Muslim obtains all that he asks from god. Therefore Muslims pray on Friday hoping that the time they pray coincides with this auspicious hour. Special prayers are performed on Friday, and a Muslim who neglects three consecutive Friday prayers is unpardonable.
Adoption of Islamic Calendar:
The era of the Hegira is the official era in Saudi Arabia, Yemen, and the principalities of the Persian Gulf. Egypt, Syria, Jordan, and Morocco use both the Muslim and the Christian eras. In all Muslim countries, people use the Muslim Era in private, even though the Christian Era may be in official use. Some Muslim countries have made a compromise on this matter. Turkey, as early as 1088 AH (AD 1677), took over the solar (Julian) year with its month names but kept the Muslim Era. March 1 was taken as the beginning of the year (commonly called marti year, after the Turkish word mart, for March). Late in the 19th century, the Gregorian calendar was adopted. In the 20th century, Pres. Mustafa Kemal Atatürk ordered a complete change to the Christian Era. Iran, under Reza Shah Pahlavi (reigned 1925-41), also adopted the solar year but with Persian names for the months and keeping the Muslim Era. March 21 is the beginning of the Iranian year. Thus, the Iranian year 1359 began on March 21, 1980. This era is still in use officially.
The Designated Months:
The first day of the month Muharram is set as the beginning of the year. The second Caliph, Umar I, set this according to the verse of Quran that designates July 16, 622 AD as the first day of the Hegira. Ramadhan, the ninth month, is observed throughout the Muslim world as a month of fasting. This particular month was designated because it is the month during which Muhammad received the first of the Quran's revelations. Muslims must see the New Moon with the naked eye before they can begin their fast.
Should the New Moon prove to be invisible, then the month Sha'ban, immediately preceding Ramadan, will be reckoned as 30 days in length, and the fast will begin on the day following the last day of this month. Ramadan is followed by the day of Id-ul-Fitr, the festival that celebrates the breaking of the fast with special prayers and festivities.
Dhu-al-Hijjah is reserved for pilgrimages to Mecca and Medina, known as the Haj. Devout Muslims desire to go on the Haj pilgrimage at least once in a lifetime. 'Id al-Adha' or the feast of sacrifice, marks the end of the pilgrimage.