Zeller's Congruence is a mathematical algorithm used to determine the day of the week for any given date. It calculates the day based on the year, month, and day, using a specific formula that involves adjustments for months and leap years. The result is a number that corresponds to a day of the week, with 0 representing Saturday, 1 for Sunday, and so on. This method was developed by Christian Zeller in the 19th century and is particularly useful for historical dates or dates far in the future. It simplifies the process of finding the weekday without needing a calendar, making it a valuable tool in both mathematics and computer science.