(Original source http://www.pithemovie.com )

Pi is both the sixteenth letter of the Greek alphabet and the symbol which represents the world's oldest mathematical mystery: the ratio of a circle's circumference to its diameter.
The earliest known written record of the ratio comes from 1650 BCE Egypt, where a scribe calculated the value to be 3.16 (a mere 1% off the true value). Although now, we have methods to calculate the digits of pi (3.1415...) its exact value remains a mystery.
Since 1794, when it was proven that Pi was both irrational and infinite, people have been searching for a pattern in the endless string of numbers.
As it turns out, Pi can be found everywhere, from astronomy to probability to the physics of sound and light. To date it has been calculated to over 51 billion digits, so far with no discernible pattern emerging from its numbers. In fact, the first time that the sequence 123456789 appears, it is over 500 million digits into the ratio. Calculating the digits to millions of decimal places is now used to test computers for bugs in hardware and software (which is how Intel's Pentium found a chip bug a few years ago).
If you prefer to calculate Pi the old fashioned way, you're not alone. Hundreds of clubs have been formed to celebrate and calculate the ratio, many of which require a good deal of sequence memorization in order to join. The current world record for pi memorization was made in 1995, when a Japanese man recited 42,000 digits from memory in just over nine hours.

pi=Circumference/Diameter

Pi is the ratio between the circumference and diameter of a circle.
It seems that all people with a written language, no matter at what point in history, had at least a fundamental knowledge of pi. Degrees of accuracy have changed over time, but little else about pi has.
Pi can also be rendered as 22/7 or 377/120, but is most commonly considered 3.14. Making the number more mysterious is that no one has ever been able to locate a repeating pattern in the decimal sequence. Most decimals repeat a pattern eventually (8.2349574623879879879879, for example), but despite having taken pi out to literally hundreds and thousands of places, no pattern has emerged -- indeed, it is thought to be a truly infinite number.
Through Pi we can thus witness the Macrocosm in the Microcosm; the Infinite within the Finite. It also adds mathematical substance to the role of the circle as a symbol of Infinity.

Mysterious relation between PI, Gregorian and Islamic calendars
(Original source http://fakir60.tripod.com/calendar.htm
The year is the period taken by the Earth to complete an orbit around the Sun and, again, there are a number of ways in which this can be measured. But for calculating a calendar that is to remain in step with the seasons, it is most convenient to use the tropical year, since this refers directly to the Sun's apparent annual motion. The tropical year is defined as the interval between successive passages of the Sun through the vernal equinox (i.e., when it crosses the celestial equator late in March) and amounts to 365.242199 mean solar days.
The tropical year and the synodic month (lunar month) are incommensurable, 12 synodic months amounting to 354.36706 days, almost 11 days shorter than the tropical year.
The Muslim Era is computed from the starting point of the year of the emigration (Hegira); that is, from the year in which Muhammad, the prophet of Islam, emigrated from Mecca to Medina, AD 622. The second caliph, 'Umar I, who reigned 634-644, set the first day of the month Muharram as the beginning of the year; that is, July 16, 622.
The years of the Muslim calendar are lunar and always consist of 12 (lunar) months, each month beginning approximately at the time of the New Moon. The months are alternately 30 and 29 days long except for the 12th, Dhu al-Hijjah, the length of which is varied in a 30-year cycle intended to keep the calendar in step with the true phases of the Moon. In 11 years of this cycle, Dhu al-Hijjah has 30 days, and in the other 19 years it has 29. Thus the year has either 354 or 355 days. No months are intercalated, so that the named months do not remain in the same seasons but retrogress through the entire solar, or seasonal, year (of about 365.25 days) every 32.5 solar years.
Now the following equation shows the in time relation between the Islamic year, the Gregorian year and the transcendent magical number Pi =3,1415.... It's to say that also history and dates are mathematically linked to each other. The conversion is usually made by special software. Try the below equation to see the accuracy of the results and control the precision with a Hegira/Gregorian converter.
622 AD is the year of Hegira. If you are in the second half of the year use 622.
Use 622,5 if you are in the first half.
Pi = 3,14159265.....