LUNAR V. SOLAR CALENDAR
A Lunar month may vary from 29.26 to 29.80 days. The average Lunar month is 29.530588 days or slightly more than 29.5 days. Twelve average Lunar months are equal to 354.3670 days while the solar year is 365.2422 days. Therefore, the Islamic Calendar gains ground in relation to the Solar Calendar at the rate of about 11 days a year or about one-year in every 33 years.
Since no effort is made to link the Islamic Calendar with solar year the Islamic months do not correspond with particular season.
A Lunar month is not less than 29 days or more than 30 days. But in solar calendar the days of months vary from 28 to 31.
A Lunar year is not less than 354 days or more than 355 days. In fact, a Lunar year is equal to 354 days 8 hours and 34 seconds but for general calculation 34 seconds are omitted. Therefore the Islamic Calendar will gain one day in 2541 years.
30 YEARS CYCLE
In Islamic Calendar a cycle of 30 years, which is called, a "small cycle" is equal to 10631 days.
In these 30 years 19 years are of 354 days while 11 years are of 355 days. In this 30 years cycle the 29/30 days duration of the month will be repeated in the same order, as they were 30 years ago.
Seven "small cycles" or 210 years are called a "big cycle" in which not only the duration of the months is repeated in the same order but the days of week also fall in the same order.
The small and big cycles of the Islamic Calendar are devised to compute the Islamic dates into Solar Calendar and vice versa.