Day of the Week Calculator
Adjust the inputs below. Results update as you type.
How it works
Uses the Tomohiko Sakamoto or Zeller's congruence algorithm to compute the day of the week for any Gregorian date. Julian calendar dates before October 15, 1582 follow different rules. Useful for historical research, event planning, and curiosity about birthdays or anniversaries. The Gregorian calendar was adopted at different times in different countries.
Input guidance
- Confirm date/time/unit settings before comparing outputs.
- If a task has optional fields, run both with and without them to understand impact.
- Save scenario variants when making practical decisions.
The formula
Algorithms like Zeller's congruence compute the weekday for any date directly from the day, month, and year using modular arithmetic.
Worked example
July 4, 1776 was a Thursday, and January 1, 2000 was a Saturday — both found from the date alone.
More examples to test
- Quick-pass example: run default assumptions for a first estimate.
- Refined example: update one assumption at a time to isolate impact.
How to interpret results
Use outputs as operational estimates and pair them with local constraints, business rules, or provider requirements.
When this can be inaccurate
Utilities can miss local policy details, special-case rules, and environment-specific constraints.
Change history
- July 2026: Quality-reviewed for publication with formula checks and explanatory copy updates.
Site-wide corrections also appear on the corrections log.
Find productivity tools for everyday workflows
General recommendation, not professional advice.
Frequently asked questions
How can you know the weekday of any date?+
Formulas like Zeller's congruence map a calendar date to a weekday using fixed rules, accounting for leap years and month lengths.
Does it work for historical dates?+
Yes, within the Gregorian calendar. Very old dates may need adjustment for the Julian-to-Gregorian switch in different countries.
Why would I need this?+
Planning events, checking which day a birthday falls on in a given year, or settling trivia about past and future dates.
Why are outputs slightly different from another tool?+
Different tools can use different rounding rules, default assumptions, and treatment of edge cases.
How can I improve estimate reliability?+
Use verified inputs from real records and re-run calculations after major assumption changes.