Day of the Year Calculator
Day number of a date, and the date of a day number.
Day of the Week Calculator
Find what day of the week any date falls on, from the year 1000 to 9999. The day of the week calculator also shows that date's weekday in each of the next 10 years.
Runs in your browser. Your data is not uploaded.
How to use the Day of the Week Calculator
- Choose a Date. It starts at today.
- Change the date to any day in the past or future.
- Press Calculate. The weekday appears with the full date, and a table shows the weekday of that date in each of the next 10 years.
How it works
The calculator counts the days between your date and January 1, 1970, a Thursday, and takes the remainder after dividing by 7:
weekday = (days since 1970-01-01 + 4) mod 7 (0 = Sunday)
Days are counted in the proleptic Gregorian calendar, with leap years included.
Examples
- January 1, 2000 was a Saturday.
- July 20, 1969, the first Moon landing, was a Sunday.
- Christmas 2026 falls on a Friday.
January 1, 2000 in the calculator
January 1, 2000 comes out as a Saturday, day 1 of the year but ISO week 52, because that week belongs to 1999. The table below the result lists the weekday of the same date for the next 10 years. This example is one of the automated tests for the tool, so a code change that gives a different answer fails the test run.
Why dates move one or two weekdays each year
365 days is 52 weeks and 1 day, so a date usually falls one weekday later the next year. After February 29 in a leap year it moves by two. The pattern of weekdays repeats every 400 years.
Limitations
- Uses the Gregorian calendar for all dates. For dates before 1582, historical records may use the Julian calendar, which differs by several days.
- A February 29 date is shown as February 28 in years without that day.
Sources
- ISO 8601: Date and time format, International Organization for Standardization. The ISO date format and ISO week numbers shown with each result.
Frequently asked questions
What day of the week was I born?
Enter your date of birth and the calculator shows the weekday.
What day was January 1, 2000?
A Saturday.
Does this work for historical dates?
For dates after 1582 in countries using the Gregorian calendar, yes. Earlier records often used the Julian calendar.
Related tools
Often used together with Day of the Week Calculator.
Leap Year Calculator
Whether a year is a leap year, with nearby leap years.
Age Calculator
Gives an exact age in years, months, and days from a date of birth.