Day of the Week Calculator Guides
Step by step guides with worked examples for the Day of the Week Calculator.
Common Day of Week Mistakes
The mistakes people make with day of the week calculations, including the leap year rule, the January reduction and conflicting week conventions.
Read the guide →How to Find the Day of the Week for Any Date
How to work out the weekday for any date using Sakamoto's algorithm, why January and February are treated specially, and how to verify the answer.
Read the guide →ISO Week Numbers Explained
How ISO 8601 week numbers are defined, why 1 January sometimes falls in week 53, and why Europe and America disagree about week starts.
Read the guide →Leap Year Rules Explained
Why leap years follow the 4, 100 and 400 rule, which years are leap years, and why 1900 was not a leap year but 2000 was.
Read the guide →Using Weekdays for Planning and Deadlines
Why knowing the weekday of a deadline matters for notice periods, filing dates, contracts and shift patterns, and how working day calculations go wrong.
Read the guide →