What Weekday and Week Number Is a Date?

This tool breaks a date into the attributes people actually ask about: weekday, week of the year, day of the year and whether the year is a leap year. Enter YYYY-MM-DD.

Open the tool：Date Info (Weekday, Week, Leap Year)

Weekday, week number and leap year

Scheduling a meeting means avoiding a weekend, so you check the weekday first. A weekly report may need a week number, and annual leave planning depends on whether the year has 365 or 366 days. All of that comes back from one input, with no calendar app needed.

It is also handy for checking a deadline that a supplier quoted as a date, or for confirming that a public holiday falls on a Monday this year and creates a long weekend. Check before you build a rota or book tickets, and rescheduling later becomes unnecessary.

Confirm which weekday a date falls on
Find the week and day of the year
Check leap-year status and the length of February

The week algorithm and leap-year rule

Week numbers here are not ISO-8601. Week 1 starts on 1 January, weeks run in seven-day blocks aligned to Sunday, and the number is the day of year plus the 1 January weekday offset, divided by seven and rounded up. ISO instead starts on Monday and calls the week containing 4 January week 1.

Leap years follow the Gregorian rule: divisible by 4 and not by 100, or divisible by 400. So 2000 is a leap year and 1900 is not; 2024 is and 2100 is not. The weekday comes from the date parsed in your browser's time zone: a date-only input is read as UTC midnight, so it matches the date in UTC+8 but can fall a day earlier west of UTC.

Weeks run in seven-day blocks from 1 January
Leap years use the Gregorian rule, including centuries
A date-only input is not shifted by the time zone

Where it differs from ISO

ISO week numbers are standard in project management and European business reporting, so a mismatch with this tool is expected rather than an error. International teams should name the convention in the document, or write the date range instead of week 43.

The tool covers a single date only: no date difference, business days, lunar calendar, solar terms or holidays. For a lunar date or festival, use an almanac; for an ISO week number, use a tool or language library that implements isocalendar.

ISO and simple week numbers can differ by one
Lunar dates and festivals need an almanac
Cross-time-zone work: confirm which zone applies

FAQ

Why does the week number differ from my calendar?

Week 1 starts on 1 January here, in seven-day blocks. ISO-8601 starts on Monday and defines week 1 as the week containing 4 January. The two can differ by one around the turn of the year: 25 October 2026 is week 44 here and week 43 in ISO.

Is the weekday local?

The weekday comes from the date as parsed in your browser's time zone. A date-only input is read as UTC midnight, so it agrees with the date in UTC+8 but can fall a day earlier west of UTC; a time component can shift the echoed date too. State the time zone when working across borders.

How is a leap year decided?

By the Gregorian rule: divisible by 4 and not by 100, or divisible by 400. So 2000 is a leap year, 1900 is not, 2024 is and 2100 is not. It only affects 29 February and the 366-day length, not the weekday calculation.