📆 Day of Week Calculator

🔧 Operation

📅 Date

Years -9999 through 9999 are supported. Negative years are BCE under the proleptic Gregorian calendar.

📅 Anchor date + count

Lists the next or previous occurrences of the anchor's weekday. The anchor date itself is NOT included in the output.

📅 From → To

Distance is signed: negative when "To" is earlier than "From". Zero days means both dates are the same.

📤 Result

{{ primaryOutput }}

{{ weekdayResult.isWeekend ? '🏖️ Weekend' : '💼 Weekday' }}

ISO date
{{ weekdayResult.isoDate }}
Day of week
{{ weekdayResult.dayOfWeek }}
Day index (Sun=0)
{{ weekdayResult.dayIndex }}
Day index ISO (Mon=1)
{{ weekdayResult.dayIndexISO }}
ISO week number
{{ weekdayResult.weekNumber }}
Day of year
{{ weekdayResult.dayOfYear }}
Days in month
{{ weekdayResult.daysInMonth }}

📊 Weekday occurrences

Anchor date
{{ scanResult.from }}
Weekday
{{ scanResult.dayOfWeek }}
Count requested
{{ scanResult.count }}
Direction
{{ scanResult.count > 0 ? 'Forward' : 'Backward' }}

Dates

{{ scanResult.occurrences.join(', ') }}

📏 Date distance

From
{{ distanceResult.from }}
To
{{ distanceResult.to }}
Days
{{ distanceResult.days }}
Weeks
{{ distanceResult.weeks }}
Remainder days
{{ distanceResult.weekRemainder }}
Same weekday?
{{ distanceResult.sameWeekday ? 'Yes' : 'No' }}
⚠️

ℹ️ About Day of Week Calculator

What this tool does

The Day of Week Calculator finds the weekday for a calendar date and exposes the companion values people usually need when checking schedules, birthdays, deadlines, logs, and historical dates. It answers three related questions on the proleptic Gregorian calendar:

  • Weekday lookup - enter a year, month, and day to get the weekday name, ISO week number, day of year, month length, and weekend/weekday status.
  • Weekday scan - list the next or previous occurrences of the same weekday from an anchor date; positive counts scan forward and negative counts scan backward.
  • Date distance - measure the signed calendar-day distance between two dates, including whole weeks and remaining days. A multiple-of-7 distance means both dates land on the same weekday.

Calendar definition

All math uses the proleptic Gregorian calendar — the modern leap-year rule (divisible by 4 except centuries not divisible by 400) extended backward in time even for dates before its 1582 adoption. This makes results consistent for software, APIs, and data validation, but it is not the same as asking which local civil calendar a country actually used on a historical date.

BCE input uses astronomical year numbering: year 0 is 1 BCE, -1 is 2 BCE, and so on. Dates are treated as date-only values, not moments in a timezone, so daylight saving time and local clock offsets do not change the result.

ISO 8601 week numbering

The week number returned is the ISO 8601 week — the week is defined by its Thursday, weeks start on Monday, and the first week of a year is the one containing the year's first Thursday (equivalently, the one containing January 4). A side effect: January 1 can belong to ISO week 52 or 53 of the previous year, and December 31 can belong to ISO week 1 of the following year.

dayIndex is the conventional 0-based Sunday-first index (0 = Sunday … 6 = Saturday). dayIndexISO is the ISO 8601 Monday-first index (1 = Monday … 7 = Sunday). Both are returned by the API for convenience.

Practical examples

Question Input Result
What day is May 26, 2026?2026-05-26Tuesday, ISO week 22, day 146 of 2026.
What weekday was January 1, 2000?2000-01-01Saturday; it belongs to ISO week 52 of 1999.
What weekday was the Apollo 11 landing?1969-07-20Sunday.
When are the next four Tuesdays?Scan 2026-05-26, count 42026-06-02, 2026-06-09, 2026-06-16, 2026-06-23.
Are two dates the same weekday?Distance 2026-05-26 to 2026-06-2328 days = 4 weeks, so both dates are Tuesdays.

Common use cases

  • Scheduling: find which weekday a meeting, deadline, release, holiday, or travel date falls on.
  • Recurring events: generate the next or previous set of matching weekdays for payroll, invoices, rotations, or maintenance windows.
  • Birthday and anniversary checks: answer "what day was I born?" or count how many full weeks separate two dates.
  • Historical research: compare dates using one consistent Gregorian rule, while remembering that many regions used Julian or local calendars before adoption.
  • Calendar-library testing: cross-check ISO week numbers, leap days, and same-weekday distances against a deterministic implementation.

Limits and interpretation

  • Supported years are -9999 through 9999; invalid dates such as February 30 are rejected instead of rolled into the next month.
  • Weekday scans are limited to 366 occurrences in either direction to keep output readable and API responses bounded.
  • The distance mode counts calendar days between date-only values. It does not calculate business days, working hours, timezone offsets, or inclusive/exclusive date ranges for payroll rules.
  • For historical dates before local Gregorian adoption, the result is a proleptic Gregorian answer. It may differ from a Julian-calendar date recorded in a historical source.

Standards and source references

API Usage Example:

POST /api/tools/time/day-of-week
Content-Type: application/json

{
    "operation": "weekday",
    "year": 2026,
    "month": 5,
    "day": 26
}
Ready