Calendar calculations involve determining dates and time intervals using various calendars, such as the Gregorian or Julian calendars. These calculations can include finding the day of the week for a specific date, calculating the number of days between two dates, or determining leap years. To perform calendar calculations, one may use algorithms or simple counting methods. For example, to find the day of the week for a given date, you can apply the Doomsday rule, a mental algorithm developed by John Horton Conway. Understanding these calculations is essential for planning events, scheduling, and historical research.