pA festival falling on different dates of a Gregorian calendar in different years is a common feature of all religions. For Hindus, the Holi Festival may occur on any day from March 1 to March 29; for Muslims, the Id may happen on any day of the year; whereas for Christians, the Easter Sunday may occur from March 22 to April 25, or as celebrated by some orthodox churches, from April 4 to May 8. This does not happen arbitrarily, but there is some logic behind this.brBefore going through the chapters, one must know that simplicity is not Natures nature. Though Nature follows a set of rules, it does not like round numbers. Take the example of the ratio between the circumference of a circle to its diameter. It is denoted by the Greek Letter Pi π which is an irrational number, i.e., its value cannot be expressed exactly as a fraction or decimal, though one can do approximations, like 227 or 355113.brIt would have been wonderful if we have exactly 4 weeks in a month. Counting weeks could have been so much easier. Or each month could have been only 30 days, a year of 360 days, and of course, no leap year. A lot better would have been a year of 100 days and 10 months in a year. But it is not. And therefore, I am writing this book intending to make the subject easy to understand. You may use your discretion in case of any difference of opinion or if you find any inconsistency which could have crept in inadvertently while collecting facts from different sources. I have also used fgUnh Hindi spellings for some words for Indian readers.p