This book will explain the mathematical details behind the Jewish calendar and Easter calculation with a mathematical proposal to fix the Jewish Calendar. In addition, we will explore the mathematics behind the Mayan calendar, Islamic calendar, and our everyday calendar, the Gregorian calendar. First, I will demonstrate how the Jewish calendar works, and how to convert dates between the Jewish calendar to the Gregorian/Julian calendar. I will then explain the Easter calculation and demonstrate how to use the methods from the Easter calculation to fix the Jewish calendar so that the Jewish calendar stays consistent with the solar cycle. In 1582, there were two competing proposals to reform the Julian calendar. I will explain the mathematics behind the two proposals. I will explain the proposal that placed Washington, DC on the 77th meridian.