Day of the Week Calculator
Find which day of the week any date falls on, in the past or the future.
Date
Day of week
What is a Day of the Week Calculator?
A Day of the Week Calculator tells you which weekday any given date falls on. Enter a date — past, present, or future — and it returns the day of the week (Monday, Tuesday, etc.), the day of the year, the week of the year, and whether the year is a leap year. People use it to look up what day of the week they were born on, to check what day a holiday or anniversary will fall on, to verify historical dates, or to plan events. Instead of trying to recall or calculate the weekday yourself (which requires knowing the Doomsday algorithm or having a reference), you get the answer instantly with supporting calendar details.
How to find the day of the week manually (Zeller's formula)
One common manual method is Zeller's Congruence, which works for Gregorian calendar dates. For a given date with month m, day d, and year y, the formula is: h = (d + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ − 2J) mod 7, where h is the day of the week (0 = Saturday, 1 = Sunday, etc.), J is the century (year / 100), K is the year within the century (year % 100), and month m is adjusted so March = 3, April = 4, ..., January = 13, February = 14 of the previous year. For example, for January 1, 2025: J = 20, K = 24 (since January is treated as month 13 of the previous year, so the year becomes 2024), m = 13, d = 1. Plugging into the formula gives h = 4, which is Wednesday. The calculator does all this math for you instantly.
Example calculation
Let's find the day of the week for July 4, 2025. Enter July 4, 2025 into the date picker and click Calculate. The result shows: Weekday: Friday. It also tells you that July 4, 2025 is day 185 of the year (31 + 28 + 31 + 30 + 31 + 30 + 4 = 185), falls in week 27 of the year, and 2025 is not a leap year (2025 is not divisible by 4). This extra information can be useful for academic calendars, fiscal year planning, or just satisfying your curiosity about where a date falls in the annual cycle.
Common mistakes
- Using the wrong calendar system — Dates before September 1752 may use the Julian calendar in some contexts. This calculator uses the Gregorian calendar.
- Forgetting leap year rules — A year divisible by 100 is not a leap year unless also divisible by 400. The year 1900 was not a leap year; 2000 was.
- Confusing AM/PM in date boundaries — The day of the week for a given date is the same regardless of time of day, but being off by one day at midnight can cause errors.
- Assuming the week starts on Monday — Different cultures start the week on different days. This tool uses your browser's locale settings.
- Misreading the day-of-year number — Day of year starts at 1 for January 1. A result of day 365 means December 31 in a non-leap year.
Frequently asked questions
Does it work for historical dates?
Yes, for dates in the standard Gregorian calendar used today. It works for any date from 1582 onward and for many dates before that using the proleptic Gregorian calendar.
How do I find the day of the week for a past date?
Simply enter the date in the date picker. The calculator instantly shows the weekday, day of the year, week of the year, and whether it is a leap year.
Can I find the day of the week for a future date?
Yes, enter any future date. The calculator works equally well for past and future dates, making it useful for planning events far in advance.
What is the day of the year?
The day of the year is a number from 1 to 365 (or 366 in a leap year) that represents how many days have passed in the current year. January 1 is day 1, and December 31 is day 365.
Does this tool account for leap years?
Yes, the calculator checks whether the year is a leap year and adjusts the day-of-year count and week-of-year calculations accordingly.
How accurate is the weekday result?
The result is 100% accurate for all dates in the Gregorian calendar. The underlying date logic follows the standard calendar rules used worldwide.