How To Calculate The Day Of The Week In Excel

Enter a date to calculate the corresponding day of the week in Excel

How to Calculate the Day of the Week in Excel: Complete Guide

Master Excel’s date functions to determine days of the week with precision

Calculating the day of the week from a given date is one of Excel’s most powerful yet underutilized features. Whether you’re analyzing historical data, planning schedules, or validating date entries, this skill can save hours of manual work. This comprehensive guide covers everything from basic formulas to advanced techniques.

Why Calculate Days of the Week?

• Automate schedule creation for shift workers
• Analyze patterns in time-series data
• Validate date entries in forms
• Calculate business days between dates
• Generate dynamic reports based on weekdays

Key Excel Functions

• WEEKDAY() – Returns day number (1-7)
• TEXT() – Formats date as day name
• CHOOS() – Converts numbers to day names
• DATE() – Creates dates from components
• TODAY() – Gets current date

Basic Methods to Calculate Day of Week

Method 1: Using WEEKDAY Function

The WEEKDAY function is the most straightforward approach:

=WEEKDAY(serial_number, [return_type])
                

Return Type	Description	Example Output
1 or omitted	1 (Sunday) through 7 (Saturday)	For 7/20/1969: 1 (Sunday)
2	1 (Monday) through 7 (Sunday)	For 7/20/1969: 7 (Sunday)
3	0 (Monday) through 6 (Sunday)	For 7/20/1969: 6 (Sunday)

Example usage:

=WEEKDAY("7/20/1969")  // Returns 1 (Sunday in default system)
=WEEKDAY("7/20/1969", 2)  // Returns 7 (Sunday when Monday=1)
                

Method 2: Using TEXT Function

The TEXT function formats dates as text:

=TEXT(date, "dddd")  // Returns full day name (e.g., "Monday")
=TEXT(date, "ddd")   // Returns abbreviated day (e.g., "Mon")
                

Example:

=TEXT("7/20/1969", "dddd")  // Returns "Sunday"
=TEXT(TODAY(), "ddd")        // Returns current day abbreviation
                

Advanced Techniques

Custom Day Numbering Systems

For specialized applications where you need custom day numbering:

=MOD(WEEKDAY(A1)-1, 7)+1  // Always returns 1-7 with Monday=1
                

Array Formulas for Multiple Dates

Process entire columns of dates with array formulas (Excel 365+):

=TEXT(A1:A100, "dddd")  // Returns array of day names
                

Zeller’s Congruence Algorithm

For historical dates before 1900 (Excel’s date limit), implement Zeller’s Congruence:

=MOD(A1,7)+1  // Where A1 contains Zeller's calculation
                

Method	Pros	Cons	Best For
WEEKDAY()	Simple, built-in	Limited to post-1900 dates	Most common scenarios
TEXT()	Human-readable output	Not numeric for calculations	Display purposes
CHOOS()	Customizable day names	Requires WEEKDAY as input	Multilingual applications
Zeller’s	Works for any date	Complex implementation	Historical date analysis

Practical Applications

Business Day Calculations

Calculate working days between dates excluding weekends:

=NETWORKDAYS(start_date, end_date)
                

Dynamic Scheduling

Create schedules that automatically adjust based on days:

=IF(WEEKDAY(TODAY())=7, "Weekend", "Weekday")
                

Date Validation

Ensure dates fall on specific days:

=IF(WEEKDAY(A1)=3, "Valid Wednesday", "Invalid Day")
                

Conditional Formatting

1. Select your date range
2. Go to Home > Conditional Formatting > New Rule
3. Use formula: =WEEKDAY(A1)=7 to highlight Sundays
4. Set your preferred formatting

Common Errors and Solutions

#VALUE! Errors

Cause: Excel doesn’t recognize your date format

Solution: Use DATEVALUE() to convert text to dates:

=WEEKDAY(DATEVALUE("20/07/1969"))
                

Incorrect Day Numbers

Cause: Different return_type parameters

Solution: Always specify the return_type explicitly:

=WEEKDAY(A1, 2)  // Explicitly use Monday=1 system
                

1900 vs 1904 Date Systems

Excel for Mac defaults to 1904 date system. Check with:

=INFO("system")  // Returns "mac" or "pc"
                

Convert between systems with:

=IF(INFO("system")="mac", A1+1462, A1)  // Convert to 1900 system
                

Historical Context and Algorithms

The calculation of days of the week has fascinated mathematicians for centuries. The modern Gregorian calendar, introduced in 1582, forms the basis for Excel’s date system. Understanding the underlying algorithms can help troubleshoot edge cases.

Modular Arithmetic Basics

Day calculation relies on modulo 7 arithmetic because weeks cycle every 7 days. The key insight is that:

(date - reference_date) MOD 7 = day_difference
                

Excel’s Date Serial Number

Excel stores dates as serial numbers where:

• 1 = January 1, 1900 (Windows) or January 1, 1904 (Mac)
• Each subsequent day increments by 1
• Time is stored as fractional days

Convert to days since epoch:

=MOD(A1, 7)  // Where A1 contains a date serial number
                

Leap Year Considerations

Leap years add complexity to date calculations. The rules are:

1. Years divisible by 4 are leap years
2. Except years divisible by 100 are not leap years
3. Unless also divisible by 400, then they are leap years

Excel handles this automatically, but for custom calculations:

=IF(OR(MOD(YEAR(A1),400)=0, AND(MOD(YEAR(A1),4)=0, MOD(YEAR(A1),100)<>0)), 1, 0)
                

Performance Optimization

For large datasets with thousands of dates, optimization becomes crucial:

Volatile vs Non-Volatile Functions

Function	Volatility	Impact	Alternative
TODAY()	Volatile	Recalculates constantly	Use fixed date or manual refresh
NOW()	Volatile	Slows large workbooks	Use static timestamps
WEEKDAY()	Non-volatile	Efficient for calculations	Preferred for day calculations

Array Processing

For Excel 365+, use dynamic arrays to process entire columns at once:

=BYROW(A1:A1000, LAMBDA(date, WEEKDAY(date)))
                

Power Query Alternative

For datasets over 100,000 rows, use Power Query:

1. Load data to Power Query Editor
2. Add custom column with formula: Date.DayOfWeek([DateColumn])
3. Load back to Excel

Authoritative Resources

For deeper understanding, consult these official sources:

• National Institute of Standards and Technology (NIST) – Time and Frequency Division: Official timekeeping standards that underpin Excel’s date calculations
• Mathematical Association of America – Gregorian Calendar Mathematics: Academic treatment of calendar algorithms
• Microsoft Office Support – Date and Time Functions: Official documentation for Excel’s date functions