Day of the WeekFree online date & time calculator — instant results, no sign-up
Find out which day of the week any date falls on.
About the Day of the Week
The Day of the Week calculator identifies the weekday for a selected calendar date and also reports its day-of-year position and an approximate week number. For the default date, 2026-10-23, the results are Friday, day 296 of the year, and approximate week 43.
- Date
- The calendar date to check, entered as year-month-day (YYYY-MM-DD), with a default of 2026-10-23.
Formula and method
For a Gregorian date, a standard weekday formula is h = (d + floor(13(m + 1)/5) + K + floor(K/4) + floor(J/4) + 5J) mod 7, where January and February are treated as months 13 and 14 of the preceding year. Map h = 0, 1, 2, 3, 4, 5, 6 to Saturday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday. Day of year: D = d + sum(L_i for i = 1 through M − 1). Approximate week number: W = ceil(D/7). For 2026-10-23: h = (23 + 28 + 26 + 6 + 5 + 100) mod 7 = 6, so the weekday is Friday; D = 273 + 23 = 296; W = ceil(296/7) = 43.
- Y, M, dThe entered calendar year, month number, and day of the month.
- y, mThe adjusted year and month for the weekday formula: y = Y − 1 and m = M + 12 for January or February; otherwise y = Y and m = M.
- KThe year within the adjusted century: K = y mod 100.
- JThe adjusted century number: J = floor(y/100).
- hThe weekday index, with Saturday represented by 0 and Friday by 6.
- L_iThe number of days in calendar month i of the entered year, including the leap-year adjustment for February.
- DThe day-of-year number, counting January 1 as day 1.
- WThe approximate week number obtained by grouping days of the year into seven-day blocks.
- floor, ceil, modRounding down to an integer, rounding up to an integer, and taking the remainder after division, respectively.
Validate the selected date and apply Gregorian calendar rules. Determine its weekday using a calendar algorithm such as the formula above, then count the days in all preceding months and add the day of the month. Divide that day-of-year count by seven and round up for the approximate week number shown here. Gregorian leap years are divisible by 4, except century years must also be divisible by 400.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Date1970-01-01
Results
- Day of weekThursday
- Day of year1
- Week number (approx.)1
How to use the Day of the Week
- 1Enter or select the date in the Date field; the default is 2026-10-23.
- 2Check that the year, month, and day match the date you intend to look up.
- 3Run the calculation, or read the updated results if the page calculates automatically.
- 4Read Day of week as Friday and Day of year as 296 for the default date.
- 5Read Week number (approx.) as 43, noting that it is an approximate year-based count rather than necessarily an ISO week number.
Reading your result
Day of week names the weekday on which the selected date falls: 2026-10-23 is Friday. Day of year gives the date's ordinal position within its calendar year: 296 means it is the 296th day, with 295 days completed before it. Week number (approx.) is 43 under the seven-day-block convention used here; that block covers day-of-year positions 295 through 301.
Limitations
The explanation assumes the Gregorian calendar, extended backward if historical dates are accepted; local historical calendars may give different results. The weekday formula is a standard way to reproduce the result, not a claim about the page's internal implementation. The approximate-week formula is consistent with the supplied example, but the example alone does not establish the implementation's convention. This count is not a substitute for ISO 8601 or other official week-numbering systems. A calendar date has no time zone; converting a timestamp to a date first can change the weekday depending on the zone. The page's supported year range is not specified.
Common questions
What results should I get for 2026-10-23?+
Day of week = Friday; Day of year = 296; Week number (approx.) = 43.
Does Day of year start at zero or one?+
It starts at one. For 2026-10-23, the preceding months contain 273 days, and adding 23 gives day 296.
Is Week number (approx.) an ISO week number?+
Do not assume it is. The approximate method described here groups days into seven-day blocks starting on January 1. ISO weeks start on Monday, and ISO week 1 is the week containing January 4.
How do leap years affect the results?+
February has an extra day in a Gregorian leap year, which affects day-of-year counts after February and weekday calculations. The default year, 2026, is not a leap year, so February contributes 28 days.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
Further reading
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Day of the Year
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Leap Year Checker
Check whether any year is a leap year.
Hours Between Dates
Calculate the total hours, minutes and seconds between two dates.
Results are estimates for informational purposes and are not financial, medical or legal advice.