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Date Info
Weekday, week number, day-of-year, leap status
Date Info
Enter values below, then calculate.
Date info returns a compact fact sheet for any calendar day: weekday name, ordinal day-of-year (1–366), ISO week number with ISO year, US convention week number, and whether the date falls in a leap year or common year. Enter a date once instead of jumping between separate week-number and leap-year pages — useful for logs, captions, metadata, and classroom worksheets.
What each field means
Weekday is the English name (Monday through Sunday) for the civil date in local calendar math. Day of year counts from January 1 as day 1 through December 31; leap years include February 29 and may reach day 366. ISO week follows ISO 8601: weeks start Monday, week 1 contains the first Thursday of the calendar year, and the ISO year label can differ from the calendar year near January and December boundaries.
US week uses Sunday as the first day of the week and January 1–based numbering common in US spreadsheets and some legacy reports — it can disagree with ISO week for the same date. Leap year vs common year follows Gregorian rules for the date's calendar year.
ISO week vs US week
A date in early January may belong to ISO week 52 or 53 of the prior ISO year while the calendar year just changed. Late December dates may belong to ISO week 1 of the next ISO year. US week numbering rarely uses that shift — which is why European project plans and US retail reports sometimes talk past each other.
When a contract cites week 24, ask which convention. Deep ISO navigation lives on week number; this page gives a quick dual read for one day.
Day-of-year in data and science
Log files, climate series, and orbital mechanics often index by day-of-year (DOY) instead of month/day to keep sorting simple. DOY 001 is January 1; DOY 365 or 366 is December 31. When merging datasets, confirm whether leap day is included in the vendor's schema — this tool states leap status explicitly for the year of your chosen date.
Programmer Day (day 256) and round hundreds appear as curiosities on the date pattern calculator, not here.
Everyday uses
Photo captions need Wednesday, day 142 of 2026. Payroll exports want ISO week alongside pay date. Students answer worksheet questions about leap years without memorizing tables. Developers writing unit tests pick boundary dates (December 31, February 29) and verify metadata before coding edge cases.
Pair with days until when you need remaining time to an event, not static metadata about one day.
Worked examples
Enter December 31, 2026 to see weekday, final day-of-year, ISO and US week labels, and leap status for that year. Enter January 4 to illustrate ISO week 1 vs calendar year alignment. Enter February 29 on a leap year to confirm day-of-year 60 and leap-year flag together.
A reporter filing a story dated ISO week 20 can enter the publication date here to confirm the ISO week matches the newsroom style guide before upload.
Limits
One date per calculation — no ranges. No moon phase, Julian date, or time zone offset (see converters and astro tools). Weekday-only counts across spans use weekday counter or days between dates when you need totals rather than metadata for a single day.
Spreadsheet and API alignment
Excel WEEKNUM defaults vary by return type and system settings — ISO week 1 near January can disagree with a formula your colleague uses. When a workbook column must match this page, compare one known date here first, then lock the spreadsheet convention in documentation.
Logging pipelines that emit DOY should zero-pad or fixed-width sort DOY in filenames so lexical order matches chronological order in object storage browsers.
Caption editors pairing weekday with ISO week should verify both fields on this page before publish — a correct weekday with a mismatched ISO week label is a common error when copy is assembled from two legacy sources.
Journalism and legal timestamps
News articles referencing week 20 of 2026 should confirm ISO week on the publication date via this page before upload. Legal filings that cite day-of-year in statutory references need the ordinal day for the jurisdiction's calendar — Gregorian civil dates are assumed here unless the filing specifies another system.
Archivists cataloging born-digital files sometimes embed ISO week in folder names — verify the embedded week against this page when migrating legacy archives whose creators mixed US and ISO conventions.
Quality assurance for developers
Before shipping date-picker features, spot-check three boundary dates per release: last day of year, first ISO week of year, and February 29 on a leap year. Store expected weekday, DOY, and ISO week in test fixtures sourced from this page rather than hand-calculated spreadsheets that drift over time.
Microservices that serialize events with ISO week should include the ISO year field alongside week number — week 1 in late December belongs to the next ISO year, and omitting the year breaks sort order in analytics warehouses.
Event planning metadata
Conference badges sometimes print ISO week for networking cohorts — generate badge data from date info on registration date rather than trusting the attendee's memory. Wedding websites listing the weekday for save-the-dates should recompute when the date shifts one day during venue negotiation.
Tax software QA teams maintain golden files for January 1, January 4, and December 31 of leap and common years — date info outputs become expected values in automated regression suites.
IoT and sensor filenames
Edge devices naming files with ISO week and DOY should generate both from the same UTC timestamp — mixing local DOY with UTC ISO week misaligns overnight batches near timezone boundaries.
Firmware teams document expected date-info outputs in test plans for leap-day OTA releases so field devices do not ship wrong week labels after February updates.
Related tools
Today's date for the current day snapshot. Day of week for weekday-focused lessons. Full week navigation on 2026 week numbers and the date tools hub.
Frequently asked questions
What facts does date info show?
Date info returns a one-day fact sheet: English weekday name, ordinal day-of-year from one through three hundred sixty-six, ISO week number with ISO year label, US-style week number, and whether the date falls in a leap or common year under standard Gregorian rules.
Why do ISO week and US week differ?
ISO weeks start on Monday; week one contains the first Thursday, so the ISO year can differ from the calendar year near January and December. US week numbering often starts Sunday from January first, producing different week labels for the same civil date in reports.
What is day-of-year?
Day-of-year counts January first as day one through December thirty-first. Leap years may reach day three hundred sixty-six. Scientists, climate archives, and log pipelines use day-of-year for sorting and indexing without separate month and day columns in filenames or database keys.
Does this page show moon phase or time zone?
No. Date info covers one Gregorian civil date only. Moon phase, time zone offset, Julian day number, and other scales live on astronomy tools and the converters hub. Use weekday counter or days between dates when you need spans across many days.
How is leap year determined?
Gregorian rules for the date's calendar year: divisible by four, except years divisible by one hundred unless also divisible by four hundred. The tool flags leap status explicitly so you know whether February had twenty-nine days that year when interpreting day-of-year totals.
When should I use week number instead?
Use week number hub pages to browse full ISO weeks, download calendars, and navigate year hubs with previous and next links. Date info is a quick snapshot for one day when you need weekday, day-of-year, and both ISO and US week labels together.