Weekday Finder

About the Weekday Finder

The Weekday Finder tells you the day of the week for any date — past, present, or future — along with related calendar information: day of the year (e.g., “167 of 366”), ISO week number, quarter, days remaining in the year, leap year status, and days in the month. Useful for verifying what day of the week a historical event fell on, planning future events on specific weekdays, or computing week-number-based scheduling.

The day of the week is determined by the Gregorian calendar, which has a 400-year cycle of 146,097 days (exactly 20,871 weeks). This means the calendar repeats exactly every 400 years — January 1, 2000 was a Saturday, and January 1, 2400 will also be a Saturday. The 7-day week is the oldest calendrical unit still in use, dating back to ancient Babylon and adopted by all major world calendars.

The ISO week-numbering system (ISO 8601) is the international standard for week numbering, used in manufacturing, retail (fashion weeks, sale weeks), and project management. Weeks start on Monday, and week 1 is the week containing the first Thursday of the year — equivalently, the week containing January 4. This means January 1 sometimes falls in week 52 or 53 of the previous year (e.g., January 1, 2021 was in ISO week 53 of 2020).

How It Works

The day of the week is computed by JavaScript’s Date.getDay() method, which returns 0 (Sunday) through 6 (Saturday) based on the operating system’s calendar library. The underlying algorithm uses Zeller’s congruence or a similar formula to compute the weekday from the year, month, and day, accounting for the Gregorian calendar’s leap year rules.

The day-of-year calculation is floor((date - startOfYear) / 86400000) + 1, where startOfYear is midnight on January 1 of the same year. The +1 makes January 1 day 1 (not day 0). For December 31 of a non-leap year, this gives 365; for December 31 of a leap year, 366.

The ISO week number is computed by a 4-step algorithm:

1. Find the Thursday of the current ISO week: tmp.setUTCDate(tmp.getUTCDate() + 4 - dayNum) where dayNum is the day-of-week with Monday = 1 and Sunday = 7.
2. Find January 1 of the year containing that Thursday: this is the ISO week-numbering year.
3. Compute the day-of-year of that Thursday: (tmp - yearStart) / 86400000 + 1.
4. Divide by 7 and round up: Math.ceil(dayOfThursday / 7). This is the ISO week number.

The ISO year may differ from the calendar year for dates near January 1 (which may be in week 52/53 of the previous ISO year) or near December 31 (which may be in week 1 of the next ISO year). For example, December 31, 2020 was in ISO week 53 of 2020, while January 1, 2021 was also in ISO week 53 of 2020 (because January 1, 2021 was a Friday, and the Thursday of that week was December 31, 2020).

The leap year calculation uses the standard Gregorian rule: a year is leap if divisible by 4, except years divisible by 100 are not leap unless also divisible by 400. So 2000 was leap, 1900 was not, 2100 will not be. This produces a 400-year cycle with 97 leap years (not 100), giving the average year length of 365.2425 days — close to the actual tropical year of 365.2422 days.

Worked Examples

For June 15, 2024, the calculator reports: day of week Saturday, day of year 167 of 366 (2024 is a leap year), ISO week Week 24 of 2024, quarter Q2 (Apr-Jun), days remaining in year 199, days in month 30 (June has 30 days).

For January 1, 2000 (the Y2K milestone), the calculator reports: day of week Saturday, day of year 1 of 366, ISO week Week 52 of 1999 (because January 1, 2000 was a Saturday, and the Thursday of that week was December 30, 1999, which is in ISO year 1999). The ISO year differs from the calendar year — a common source of bugs in week-based reporting.

For September 11, 2001, the calculator reports: day of week Tuesday, day of year 254 of 365, ISO week Week 37 of 2001, quarter Q3 (Jul-Sep).

For December 25, 1776 (Washington’s crossing of the Delaware), the calculator reports: day of week Wednesday — correctly handling dates before the Gregorian calendar’s adoption in Britain’s colonies in 1752. The calculator uses the proleptic Gregorian calendar for all dates, which matches historical records for most purposes.

When to Use This Tool

• Verifying the day of the week for historical events (births, deaths, battles, discoveries).
• Planning future events on specific weekdays (weekend weddings, weekday meetings, Friday launches).
• Computing ISO week numbers for manufacturing, retail, and project management schedules.
• Determining quarter boundaries for financial reporting and quarterly business reviews.
• Counting days remaining in the year for goal tracking and end-of-year planning.
• Verifying leap year status for date arithmetic (e.g., computing February 29 occurrences).
• Determining the day-of-year for environmental, agricultural, and astronomical calculations.

Limitations & Disclaimer

This tool computes the day of week, day of year, ISO week number, quarter, and leap year status using JavaScript’s Date object and the IANA timezone database. All dates are interpreted in the proleptic Gregorian calendar, which extends Gregorian rules backward to dates before 1582 — historical records from before the Gregorian adoption may use the Julian calendar and differ by 10-13 days. The ISO week algorithm uses the standard ISO 8601 definition (week starts Monday, week 1 contains the first Thursday). The calculator uses your browser’s local time zone for date interpretation; for UTC-strict date arithmetic, convert dates to UTC explicitly. The 400-year Gregorian cycle (146,097 days) means calendar patterns repeat exactly every 400 years. See our disclaimer for full terms.

Frequently Asked Questions

Last updated: July 21, 2026  ·  Author: HT99 Tools Editorial Team  ·  Reviewed by: HT99 Tools Editorial Team