How To Calculate Day Of The Week In Excel

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

Calculating the day of the week from a date is one of the most useful date functions in Excel. Whether you’re analyzing sales data by weekday, scheduling projects, or creating dynamic calendars, knowing how to extract the weekday from a date can save you hours of manual work.

In this comprehensive guide, we’ll cover:

• The built-in WEEKDAY function (modern Excel versions)
• Alternative formulas for older Excel versions
• How to return day names instead of numbers
• Practical applications with real-world examples
• Common errors and how to fix them
• Performance considerations for large datasets

Method 1: Using the WEEKDAY Function (Excel 2007 and Later)

The WEEKDAY function is the simplest way to determine the day of the week from a date in modern Excel versions. Here’s the syntax:

=WEEKDAY(serial_number, [return_type])

Parameters:

• serial_number: The date you want to evaluate (can be a cell reference or date value)
• return_type (optional): Determines the numbering system for the result

Return Type	Value	Description
1 or omitted	1-7	Numbers 1 (Sunday) through 7 (Saturday)
2	1-7	Numbers 1 (Monday) through 7 (Sunday)
3	0-6	Numbers 0 (Monday) through 6 (Sunday)

Example: To find out what day of the week July 20, 1969 (the moon landing) was:

=WEEKDAY("7/20/1969", 1)

This returns 1, which corresponds to Sunday (in return_type 1).

Method 2: Getting the Day Name Instead of Number

While WEEKDAY returns a number, you can combine it with the TEXT function to get the actual day name:

=TEXT("7/20/1969", "dddd")

This returns “Sunday”. The format code “dddd” gives the full day name, while “ddd” would return the abbreviated name (e.g., “Sun”).

Pro Tip: You can also use:

=CHOOSER(WEEKDAY(A1), "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday")

Where A1 contains your date. This gives you more control over the output format.

Method 3: For Excel 2003 and Earlier (MOD Function)

If you’re working with older versions of Excel that don’t have the WEEKDAY function, you can use this formula based on Zeller’s Congruence algorithm:

=MOD(A1,7)

Where A1 contains your date. This returns:

• 0 = Sunday
• 1 = Monday
• 2 = Tuesday
• 3 = Wednesday
• 4 = Thursday
• 5 = Friday
• 6 = Saturday

For a more complete solution that works across all Excel versions, use this formula:

=CHOOSE(WEEKDAY(A1,2),"Monday","Tuesday","Wednesday","Thursday","Friday","Saturday","Sunday")

Practical Applications

Here are some real-world scenarios where calculating the day of the week is invaluable:

1. Retail Analysis: Compare sales performance by weekday to optimize staffing and promotions.
2. Project Management: Automatically highlight weekends in Gantt charts or timelines.
3. Payroll Processing: Calculate overtime differently for weekends vs. weekdays.
4. Event Planning: Ensure events don’t fall on weekends or specific weekdays.
5. School Scheduling: Alternate class schedules based on the day of the week.

Weekday Sales Analysis Example

Day of Week	Average Sales	% of Weekly Total	Staffing Recommendation
Monday	$12,450	14.2%	Standard staffing
Tuesday	$11,890	13.6%	Standard staffing
Wednesday	$13,220	15.1%	Standard staffing
Thursday	$14,560	16.6%	Increase by 10%
Friday	$18,780	21.4%	Increase by 20%
Saturday	$15,340	17.5%	Increase by 15%
Sunday	$11,230	12.8%	Standard staffing

Common Errors and Solutions

Even experienced Excel users encounter issues with date calculations. Here are the most common problems and how to fix them:

1. #VALUE! Error:

   Cause: The cell reference doesn’t contain a valid date.

   Solution: Ensure the cell is formatted as a date (Right-click → Format Cells → Date). If importing data, use DATEVALUE() to convert text to dates.

2. Incorrect Day Calculation:

   Cause: Using the wrong return_type in WEEKDAY.

   Solution: Remember that return_type 1 starts with Sunday=1, while return_type 2 starts with Monday=1.

3. 1900 vs. 1904 Date System:

   Cause: Excel for Mac sometimes uses the 1904 date system, which can offset calculations by 4 years.

   Solution: Go to Excel Preferences → Calculation and ensure “1904 date system” is unchecked unless you specifically need it.

4. Leap Year Issues:

   Cause: February 29 in non-leap years causing errors.

   Solution: Use DATE() to construct dates instead of typing them directly: =DATE(2023,2,29) will automatically adjust to March 1, 2023.

Performance Considerations

When working with large datasets (10,000+ rows), date calculations can slow down your workbook. Here are optimization tips:

• Use Helper Columns: Calculate the weekday once in a helper column rather than repeating the formula multiple times.
• Avoid Volatile Functions: Functions like TODAY() and NOW() recalculate every time Excel does anything, which can slow performance. Use static dates when possible.
• Limit Format Changes: Applying custom number formats (like “dddd”) to entire columns is resource-intensive. Apply formatting only to visible cells.
• Use Table References: Convert your data to an Excel Table (Ctrl+T) for more efficient formula calculation.
• Disable Automatic Calculation: For very large workbooks, switch to manual calculation (Formulas → Calculation Options → Manual) and recalculate only when needed (F9).

Advanced Techniques

For power users, here are some advanced applications of weekday calculations:

1. Count Weekdays Between Dates:

   Use NETWORKDAYS() to count only weekdays (excluding weekends and optionally holidays):

   =NETWORKDAYS(start_date, end_date, [holidays])

2. Create Dynamic Weekday Names:

   Combine WEEKDAY with INDEX to create custom weekday lists:

   =INDEX({"Sun","Mon","Tue","Wed","Thu","Fri","Sat"}, WEEKDAY(A1,1))

3. Conditional Formatting by Weekday:

   Highlight all weekends in your dataset with conditional formatting using this formula:

   =OR(WEEKDAY(A1,2)>5, WEEKDAY(A1,2)=7)

4. Generate Date Sequences:

   Create a series of dates that fall on specific weekdays:

   =WORKDAY(start_date, days, [holidays])

   Where days is the number of weekdays (not calendar days) you want to add.

Official Microsoft Documentation

For the most authoritative information on Excel’s date functions, refer to Microsoft’s official documentation:

Microsoft Support: WEEKDAY function

Mathematical Foundation

The algorithms behind Excel’s date calculations are based on mathematical principles explained in detail by the National Institute of Standards and Technology:

NIST Time and Frequency Division

Frequently Asked Questions

1. Why does Excel think 1/0/1900 is a valid date?

   This is a legacy issue from Lotus 1-2-3 compatibility. Excel treats dates as serial numbers where 1 = January 1, 1900 (or January 2, 1904 in the 1904 date system). The “day zero” (0) would be December 31, 1899, so 1/0/1900 is interpreted as January 0, 1900, which Excel converts to December 31, 1899.

2. Can I calculate the day of the week for dates before 1900?

   Excel’s date system doesn’t support dates before January 1, 1900 (or January 2, 1904). For historical dates, you’ll need to use a custom formula based on Zeller’s Congruence or implement a more complex algorithm.

3. How do I handle time zones in weekday calculations?

   Excel doesn’t natively handle time zones in date functions. All calculations are based on the system’s local time. For time zone conversions, you’ll need to adjust the date by adding/subtracting the time difference before applying the WEEKDAY function.

4. Why does my weekday calculation differ from other systems?

   Different systems may use different weekday numbering (Sunday=0 vs. Monday=0) or different epoch dates. Always verify which system your data source uses. Excel’s WEEKDAY function with return_type 2 (Monday=1) matches the ISO 8601 standard.

Alternative Approaches

While Excel’s built-in functions are powerful, there are alternative methods to calculate the day of the week:

1. Power Query:

   In Excel’s Get & Transform Data tools, you can add a custom column with:

   Date.DayOfWeek([YourDateColumn], Day.Monday)

   This returns the day name directly and is very efficient for large datasets.

2. VBA User-Defined Function:

   For complete control, create a custom function in VBA:

   Function GetWeekdayName(d As Date) As String
       GetWeekdayName = Format(d, "dddd")
   End Function

   Then use =GetWeekdayName(A1) in your worksheet.

3. Office Scripts (Excel Online):

   For Excel Online users, you can create automated scripts that calculate weekdays across workbooks.

Historical Context

The calculation of weekdays from dates has a fascinating history:

• The 7-day week has been used for over 2000 years, with roots in Babylonian astronomy.
• The current weekday names in English come from Norse and Roman gods (e.g., Thursday = Thor’s day, Saturday = Saturn’s day).
• The ISO 8601 standard (which Excel’s return_type 2 follows) was established in 1988 to standardize date and time representations internationally.
• Before computers, people used mechanical devices like “perpetual calendars” or mathematical algorithms like Zeller’s Congruence (published in 1883) to calculate weekdays.

Understanding these historical contexts can help when working with legacy systems or non-standard date formats from different cultures.

Final Recommendations

Based on our experience working with Excel date functions across thousands of projects, here are our top recommendations:

1. Always use return_type 2: This matches the ISO standard (Monday=1) and is the most internationally compatible option.
2. Store dates as dates: Never store dates as text – always use Excel’s date format to ensure proper calculation.
3. Document your formulas: Add comments explaining which weekday numbering system you’re using.
4. Test edge cases: Always check your formulas with dates around leap days (Feb 29) and century changes (e.g., 12/31/1999 to 1/1/2000).
5. Consider time zones: If working with international data, document which time zone your dates represent.
6. Use tables for large datasets: Convert your data to Excel Tables for better performance with date calculations.

By mastering these techniques, you’ll be able to handle virtually any weekday calculation scenario in Excel, from simple date lookups to complex scheduling systems.