keisoku

Day of the Week Calculator

Instantly find out what day of the week any date falls on. See the weekday in your language plus the day of the year, ISO week number, and Japanese era.

Input

YYYY/MM/DD

Pick a year, month, and day to see that date's weekday along with its day-of-year, week number, and Japanese era.

Result

Enter the date you want to check.

How it works

  • Pick a year, month, and day to see whether the date falls on Sunday through Saturday, shown in the weekday name of your language. Sundays are colored red and Saturdays blue so they stand out at a glance.
  • Along with the weekday, the tool counts the day of the year starting from January 1. It gives you a quick sense of how far into the year a date is and how many days remain until year's end.
  • It displays the week number using the ISO 8601 standard. Weeks start on Monday and the first week is the one containing the year's first Thursday, an international convention used in business calendars and scheduling.
  • The matching Japanese era (Reiwa, Heisei, Showa, and so on) is calculated automatically. Even for years that span an era change, the era and year of the date are shown correctly.
  • A classic way to work out a weekday by hand is Zeller's congruence, which derives the day from the year, month, and day using modular arithmetic. This tool produces the same result instantly through built-in calendar logic.
  • Use it for any date, whether checking the weekday of a past event or finding out if a future plan lands on a weekday or weekend.