Day of the Week Calculator

Calculate the weekday for any date in history or future. Find time spans between dates, business days, and more.

Zeller's Congruence: An algorithm to calculate the day of the week for any date in the Gregorian calendar.

Enter any date to find its weekday
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Understanding Weekday Calculations

The day of the week for any date can be calculated using algorithms like Zeller's Congruence or by counting days from a known reference point. Our calculator uses JavaScript's Date object which implements these algorithms to provide accurate results for dates from year 1 to 275,760.

Zeller's Congruence Algorithm:

h = (q + floor((13*(m+1))/5) + K + floor(K/4) + floor(J/4) - 2*J) mod 7

Where:
h = day of the week (0=Saturday, 1=Sunday, ..., 6=Friday)
q = day of the month
m = month (3=March, 4=April, ..., 14=February)
K = year of the century (year mod 100)
J = zero-based century (floor(year/100))

Gregorian Calendar Rules

Aspect Description Example
Leap Year Rule A year is a leap year if it is divisible by 4, but not by 100 unless also divisible by 400 2000 was a leap year, 1900 was not
Weekday Cycle The weekdays repeat every 7 days If Jan 1 is Sunday, Jan 8 is also Sunday
Month Lengths Months have 28-31 days depending on the month January has 31 days, February has 28 or 29
Century Rule Every 400 years, the calendar repeats exactly 1/1/2000 was Saturday, 1/1/2400 will be Saturday
Doomsday Rule Certain dates always fall on the same weekday each year 4/4, 6/6, 8/8, 10/10, 12/12 always on same day

Historical Calendar Changes

1

Julian to Gregorian Transition: Different countries adopted the Gregorian calendar at different times. For dates before the adoption in each country, calculations may differ from historical records.

2

Calendar Reform of 1752: Great Britain and its colonies adopted the Gregorian calendar in 1752, skipping 11 days (September 2 was followed by September 14).

3

Leap Year Adjustments: The Gregorian calendar corrected the Julian calendar's overcounting of leap years by omitting 3 leap years every 400 years.

Practical Applications

  • Event Planning: Determine what day of the week an event will fall on
  • Project Management: Calculate business days for project timelines
  • Historical Research: Verify dates in historical documents
  • Financial Calculations: Determine settlement dates for financial transactions
  • Personal Use: Find what day your birthday will fall on in future years

Calculator Features:

  • Calculates weekday for any date from year 1 to 275,760
  • Calculates difference between two dates in days, weeks, months, and years
  • Business day calculator excludes weekends and optionally holidays
  • Add or subtract days, weeks, months, or years to a date
  • Visual calendar display for selected dates

Frequently Asked Questions

This calculator can work with dates from year 1 AD to year 275,760 using JavaScript's Date object. However, for dates before the adoption of the Gregorian calendar in different countries, the results may not match historical records due to calendar reforms.

Your birthday advances one weekday each year, and two weekdays after a leap year. For example, if your birthday was on a Monday in 2023, it will be on a Wednesday in 2024 (because 2024 is a leap year, so it advances two days).

Business days are calculated by excluding Saturdays and Sundays from the total days between two dates. When the "Exclude US holidays" option is selected, major US federal holidays are also excluded from the count.

The Doomsday rule is a method to calculate the day of the week for any date mentally. It's based on the fact that certain dates (like 4/4, 6/6, 8/8, 10/10, and 12/12) always fall on the same day of the week in any given year. This day is called the "doomsday" for that year.

For dates after October 15, 1582 (when the Gregorian calendar was first adopted), the calculator is accurate. For dates before this, it uses the "proleptic Gregorian calendar" (extending the Gregorian calendar backward), which may not match historical records in countries that adopted the Gregorian calendar later.