What Day Calculator — Find the Day of the Week for Any Date

How to Use the What Day Calculator

1. Choose your calculation mode: The calculator offers two tabs at the top. The Date Lookup tab lets you select any specific date to see its day of the week. The Days From Today tab lets you enter a number of days to calculate a future or past date relative to today. Select the tab that matches your needs.
2. Enter your input: In Date Lookup mode, use the date picker to select any date from the calendar. The picker supports keyboard input in YYYY-MM-DD format or navigation through the calendar widget. In Days From Today mode, type the number of days into the input field and select the direction using the dropdown: choose "From Now (Future)" to add days to today or "Ago (Past)" to subtract days from today.
3. Review the results: The results panel displays the day of the week in large text as the primary result. Below that, you will see additional information including the day's position within the year (day of year), whether it falls on a weekday or weekend, the ISO day number, and the number of remaining days in that year. All information updates instantly as you change inputs.
4. Explore different dates: Switch between modes at any time to explore different scenarios. Try looking up your birthday, holidays, or important historical dates. Use the Days From Today mode to find deadlines, delivery dates, or future milestones. The calculator handles all date arithmetic including leap years and month-length variations automatically.

The calculator is designed for quick, repeated lookups. Whether you need to check a single date or explore dozens of scenarios, the instant results and dual-mode interface make it efficient for any day-finding task.

What Day Calculator Formula

How Day-of-Week Calculation Works

The calculator determines the day of the week using the Gregorian calendar system. Every date maps to exactly one day of the week in a cycle of seven. The algorithm works by computing the number of days between a known reference point and the target date, then finding the remainder when divided by 7. This remainder maps directly to a day name: 0 for Sunday, 1 for Monday, through 6 for Saturday.

Variables Explained

• Date: Any calendar date in the Gregorian system. Can be in the past, present, or future. The algorithm works for any valid date within the calendar's range.
• Day Name: One of seven possibilities: Sunday through Saturday. Derived from the mathematical remainder of the date calculation.
• ISO Day Number: Monday = 1, Tuesday = 2, through Sunday = 7. This follows the ISO 8601 international standard used in business and computing.
• Day of Year: The ordinal position of the date within its year, from 1 (January 1) to 365 or 366 (December 31 in a leap year).
• N (Days From Today): The number of days to add (future) or subtract (past) from today's date when using the second mode.

Step-by-Step Example

Find what day December 25, 2026 falls on:

1. Select the Date Lookup tab
2. Enter December 25, 2026 in the date picker
3. The algorithm calculates the day index: December 25, 2026 is a Friday
4. Day of year: December 25 is the 359th day of 2026
5. ISO day number: Friday = 5
6. Type: Weekday
7. Days remaining in 2026: 365 - 359 = 6 days

For the Days From Today calculation: if today is February 20, 2026 and you enter 100 days from now, the calculator adds 100 days to get May 31, 2026. The result shows Saturday, day 151 of the year, with 214 days remaining.

Practical Examples

Example 1: Maria Plans Her Wedding Date

Maria and David are planning their wedding and want it to be on a Saturday in September 2026. Maria uses the Date Lookup mode to check several dates:

• September 5, 2026: Saturday (day 248 of the year)
• September 12, 2026: Saturday (day 255 of the year)
• September 19, 2026: Saturday (day 262 of the year)
• September 26, 2026: Saturday (day 269 of the year)

Maria quickly identifies four possible Saturdays in September 2026. She picks September 19 because it falls in the middle of the month, giving guests enough time to plan after Labor Day and before the end-of-month rush. The day-of-year information (day 262 with 103 days remaining) helps her frame the date on invitations with a poetic touch.

Example 2: Raj Calculates a Project Deadline

Raj is a project manager who needs to deliver a software release exactly 90 days from today (February 20, 2026). He switches to the Days From Today tab:

• Days entered: 90
• Direction: From Now (Future)
• Result: May 21, 2026 — Thursday
• Day of year: 141

Knowing the deadline falls on a Thursday, Raj schedules the final QA review for Monday May 18 and staging deployment for Wednesday May 20, ensuring the release happens during business hours. He uses our days from now calculator to double-check the countdown as the date approaches.

Example 3: Aisha Researches a Historical Date

Aisha is writing a history essay about the moon landing and wants to know what day of the week July 20, 1969 fell on. She uses the Date Lookup tab:

• Date: July 20, 1969
• Day: Sunday
• ISO Day Number: 7
• Day of Year: 201
• Type: Weekend

The moon landing occurred on a Sunday, which explains why so many families were able to watch the historic event live on television. Aisha includes this detail in her essay to illustrate how the timing of the event contributed to its massive cultural impact. She also checks other key Apollo mission dates to build a comprehensive timeline.

Example 4: Tom Checks His Warranty Expiration

Tom bought a laptop 365 days ago and wants to verify when his 1-year warranty started. He uses the Days From Today tab with direction set to Ago (Past):

• Days entered: 365
• Direction: Ago (Past)
• Result: February 20, 2025 — Thursday
• Day of Year: 51

Tom confirms his laptop was purchased on February 20, 2025, meaning his warranty expires today. He immediately contacts customer support to report a battery issue before the warranty lapses. The calculator saved him from missing a critical deadline by allowing instant verification of past dates.

Day of Week Reference Table

Notable Dates and Their Days

Tips and Complete Guide

Understanding the Gregorian Calendar Cycle

The Gregorian calendar repeats its day-of-week pattern every 400 years. Within those 400 years, there are exactly 97 leap years, giving a total of 146,097 days. Since 146,097 is exactly divisible by 7 (= 20,871 weeks), the entire pattern of which dates fall on which days repeats perfectly. This mathematical property means that any date calculation tool can theoretically work for any date in the Gregorian calendar's history with complete accuracy.

Practical Uses for Day-of-Week Lookups

Knowing the day of the week for a date has many practical applications. Event planners use it to ensure celebrations fall on weekends. HR departments check whether public holidays create long weekends. Real estate closings are typically scheduled on weekdays. Doctors schedule procedures on specific days for optimal staffing. Teachers plan lesson schedules around testing days. Travelers check whether their arrival day coincides with business hours at their destination. The What Day Calculator serves all these needs with a single, fast interface.

Working With Both Calculator Modes

The two modes complement each other for different workflows. Use Date Lookup when you have a specific date in mind and want to know its day name. Use Days From Today when you know a duration (like a 90-day deadline or 30-day return window) and need to find the actual calendar date it produces. You can quickly switch between modes to verify calculations from different angles. For example, if Days From Today shows that 100 days from now is May 31, you can switch to Date Lookup and enter May 31 to confirm it matches.

Day of the Week in Different Cultures

Different cultures consider different days as the start of the week. The ISO 8601 standard defines Monday as day 1, which is the convention in most of Europe, Asia, and international business. The United States and Canada traditionally start the week on Sunday, which is reflected in the JavaScript convention where Sunday is day 0. Some Middle Eastern countries consider Saturday as the first day of the week. Our calculator displays both the ISO and standard numbering systems to accommodate different conventions.

Common Mistakes to Avoid

• Confusing ISO and JavaScript day numbering: In ISO 8601, Monday is 1 and Sunday is 7. In JavaScript (and many American calendars), Sunday is 0 and Saturday is 6. Our calculator shows both to prevent confusion, but be aware of which convention your other tools use.
• Forgetting leap years in manual calculations: When mentally counting days forward or backward, people often forget to account for leap years (366 days instead of 365). The calculator handles this automatically, but if you are verifying manually, remember to check for February 29.
• Assuming the calendar was always Gregorian: The Gregorian calendar was introduced in October 1582 by Pope Gregory XIII. Dates before this used the Julian calendar, which had different leap year rules. Our calculator uses Gregorian rules for all dates, which is technically incorrect for dates before 1582 but matches the standard practice for historical date lookups.
• Time zone date shifts: A date can be different depending on your time zone. December 31 in New York is already January 1 in Tokyo. If you are looking up a date for an international event, be sure you are using the date in the correct time zone. Our time zone calculator can help with conversions.
• Mixing up "from now" and "from today": The Days From Today mode calculates from the current calendar date, not from the current moment. If it is 11 PM and you calculate 1 day from now, the result is tomorrow's date, not 24 hours from this moment. For time-level precision, use our time duration calculator.

Frequently Asked Questions