Zeller's rule is a formula used to calculate the day of the week for any given date. The formula takes into account the month, date, and year. For example, applying the formula to August 27, 2014 shows it was a Wednesday. The document provides the full Zeller's rule formula and explains how to apply it step-by-step using an example date. It also notes some key points about using the formula, such as treating March as the first month.