Calendar algorithms are methods used to calculate the day of the week for any given date. These algorithms simplify the process by using mathematical formulas or rules, allowing users to determine the day without needing a physical calendar. They often take into account factors like leap years and the varying number of days in each month. One popular example is the Zeller's Congruence, which provides a straightforward way to find the day of the week for any date in the Gregorian calendar. Other algorithms, like the Doomsday Rule, also help in quickly identifying days of the week for specific dates.