Day of the Week Calculator

What this calculator does

The Day of the Week Calculator returns the weekday for a Gregorian calendar date. It also reports the ISO weekday number, where Monday is 1 and Sunday is 7, and whether the date falls on a weekend.

An optional offset shifts the date by a whole number of days before the weekday is reported. A positive offset moves forward in time and a negative offset moves backward, which is handy for questions such as "what day is it ten days from now?" or "what weekday was it three days before this date?"

How to use this calculator

1. Enter the date in YYYY-MM-DD format, or use the date picker.
2. Optionally enter an offset in days. Leave it at 0 to use the date itself.
3. Click Calculate.

The default example is November 3, 2026, which is a Tuesday. An offset of 10 days gives November 13, 2026, a Friday. An offset of -1 gives November 2, 2026, a Monday.

Weekday formula

The calculator uses the Gregorian calendar and UTC-based date arithmetic. One way to compute the weekday is Zeller's congruence:

If month is January or February:
  month = month + 12
  year = year - 1
K = year mod 100
J = floor(year / 100)
h = (day + floor(13 × (month + 1) / 5) + K + floor(K / 4)
     + floor(J / 4) + 5J) mod 7

The value of h maps to Saturday = 0, Sunday = 1, Monday = 2, and so on. A modern implementation can use the same proleptic Gregorian rule through a UTC date object, which is what this calculator does. Both approaches produce the same weekday for valid dates.

What the results mean

The weekday name is the primary result. The ISO weekday number is useful for sorting and for systems that use Monday-first weeks. The weekend flag is true for Saturday and Sunday.

When an offset is supplied, the result panel adds the shifted date and its weekday. A day shift of +1 is always the next calendar day, even across a month or year boundary. The calculator does not skip weekends or holidays unless you select them indirectly by choosing the shifted date.

Assumptions and limitations

• Only the Gregorian calendar is supported.
• Dates must use YYYY-MM-DD and must be valid calendar dates.
• The offset must be a whole number from -100,000 to 100,000 days.
• Calculation is performed in UTC, so the weekday does not depend on the device time zone.
• The calculator does not list holidays or business-day counts.

Frequently asked questions

What weekday numbering does the calculator use?

The summary shows the ISO weekday number, where Monday is 1 and Sunday is 7. The internal JavaScript weekday index starts with Sunday at 0, but that detail is not exposed as the primary result.

How do I find the weekday for a date in the past?

Enter the historical date and leave the offset at 0. Gregorian dates before the calendar reform are interpreted using the proleptic Gregorian calendar, so results for very old dates may differ from historical local calendars.

Can I skip weekends with the offset?

No. The offset moves by calendar days, not business days. If you need the next business day, calculate the offset and then adjust manually for Saturday, Sunday, or holidays.

Source

Reference: Calculator.net — Day of the Week Calculator.