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What day of the week was a date?

Pick a date to see its weekday, day-of-year, ISO week number and more.

How Day of the week works

Day of the Week tells you which weekday any date falls on, along with two supplementary facts: the day-of-year number (from 1 on 1 January to 365 or 366 on 31 December) and the ISO week number. All three values are computed locally in the browser the moment you select a date; no query goes to a server, and the result is available offline once the page has been opened.

The weekday is determined using the Tomohiko Sakamoto algorithm, which directly computes the day index from the year, month and day values without needing to iterate through dates. The day-of-year is the sum of the days in each completed month up to the date, with an extra day added for months after February in leap years. The week number follows the ISO-8601 standard described in the week-number tool.

Written by Bastien Sulyan

How to use Day of the week, step by step

  1. Select or type the date you want to look up.
  2. Read the weekday name, day-of-year and ISO week number from the result panel.
  3. Navigate forward or backward using the arrow buttons to check surrounding dates.

Common use cases

  • You need to know on which day of the week a historical date fell for a research paper or anniversary calculation.
  • A scheduler wants to confirm whether a planned date lands on a weekday or a weekend before booking a venue.
  • A payroll administrator checks the day-of-year number to align with a day-based salary calculation.
  • A developer wants to verify what day a specific timestamp corresponds to without writing code.

Frequently asked questions

Which algorithm does the tool use to determine the weekday?

The tool uses the Tomohiko Sakamoto algorithm, a compact formula that returns the day-of-week index (0 = Sunday through 6 = Saturday) directly from the year, month and day integers. It handles the Gregorian calendar correctly for all dates after the calendar reform.

What is the day-of-year number?

It is the ordinal position of the date within its year: 1 January = 1, 31 December = 365 (or 366 in a leap year). It is used in some scheduling, logistics and accounting systems as a compact date reference.

Does this tool use the same week numbering as the week-number tool?

Yes. The week number shown here follows ISO-8601, where weeks start on Monday and week 1 is the week containing the first Thursday of the year. This is the same standard as the dedicated week-number tool.

Is the date I look up recorded anywhere?

No. The date you select stays inside the browser tab. There is no analytics event, no server call, and no local storage write for this particular lookup. Close the tab and the date is gone.

Can I look up a date more than 100 years in the past or future?

Yes, over a range far wider than any practical lookup. One caveat for distant history: the weekday is computed with the proleptic Gregorian calendar, meaning the Gregorian rule is applied backwards even to dates before the 1582 reform. For those dates the answer is the Gregorian weekday, not the Julian one people were actually using at the time.

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