The Islamic Calendar is purely a lunar calendar. It attempts to keep in phase with the appearance of the new moon. Each month is defined as beginning at the sighting of the lunar crescent. Therefore, it is currently not possible to predict when a new month will begin with very high accuracy. The moon's visibility depends on many factors including the observer's quality of eyesight and the level of air pollution.
The Islamic Calendar begins on the Hijra: July 19, 622 CE of the Gregorian Calendar, the day the prophet Muhammed traveled from Mecca to Medina. Each year in the Islamic Calendar consists of 12 months. Odd months have 30 days while even months have 29 days. On leap years, the final month is extended to 30 days. Leap years occur during years 2, 5, 7, 10, 13, 16, 18, 21, 24, 26 and 29 of a 30-year cycle. Thus, every 30 years, there are 19 normal years with 354 days and 11 years with 355 days. This gives an average of 10361/30, or 354.36666666666... days per year. Dividing by twelve gives the average number of days per month:10631/360, or 29.530555555555... This is about 3.244*10^-5 days too short. Thus, the calendar accumulates an error of about one day every 2492 years. As a purely lunar calendar, no attempt is made to keep in phase with the seasons, which shift by just under 10.9 days per year.