Zeller is a mathematical algorithm used to calculate the day of the week for any given date. Developed by Christian Zeller in the 19th century, it simplifies the process of determining whether a specific date falls on a Monday, Tuesday, or any other day. The formula takes into account the year, month, and day, making it a useful tool for both mathematicians and enthusiasts of calendar-related problems. The algorithm is particularly effective for dates in the Gregorian calendar, which is the calendar system used by most of the world today. Zeller's formula can be applied to dates ranging from the year 1583 onward, when the Gregorian calendar was first adopted. This makes it a valuable resource for historical research and date calculations in various fields.