Zeller's Theorem is a mathematical formula 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 algorithm. The formula accounts for adjustments in the calendar, such as leap years, making it applicable to both the Gregorian and Julian calendars. To use Zeller's Theorem, you assign numerical values to the month and year, then perform a series of calculations. The result corresponds to a day of the week, with values ranging from 0 (Saturday) to 6 (Friday). This theorem provides a systematic way to find the weekday for any date.