The Mathematics of the Gregorian Calendar

Book excerpt: Most of this book is based on codes assigned to each month of the year, to each year and to each century. Since Pope Gregory XIII replaced the Julian calendar with the Gregorian calendar on February 24, 1582, (and I don't know if he named after himself), the calendar has followed a cycle that makes it predictable. That's where you and I come in. The mathematics is based on an easily learned system called Modulo 7. The problems involve dates and are based an equation with involving 4 quantities. One of them will be unknown. In Task A, that will be the day of the week. If a carnival had a wheel with a spinner that had the 7 days of the week on them, wherever the spinner stops MUST be a day of the week. That is essentially modulo 7. There are no fractions. There are just seven numbers. Readers should know multiples of 7 to avoid having to look at the table.