How To Calculate Day Of Week In Excel

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

Introduction to Excel Date Calculations

Microsoft Excel stores dates as sequential serial numbers called date serial numbers. This system starts with January 1, 1900 as day 1 (or January 1, 1904 in Mac versions) and increments by 1 for each subsequent day. Understanding this system is crucial for accurate day-of-week calculations.

Excel’s date system enables powerful temporal calculations, from simple day-of-week determination to complex financial modeling with business days. The methods we’ll explore work across all modern Excel versions (2010 and later) with minor variations for legacy versions.

Method 1: Using the WEEKDAY Function

The WEEKDAY function is Excel’s built-in tool for determining the day of the week. Its syntax is:

=WEEKDAY(serial_number, [return_type])

Return Type Options:

• 1 or omitted: Numbers 1 (Sunday) through 7 (Saturday)
• 2: Numbers 1 (Monday) through 7 (Sunday)
• 3: Numbers 0 (Monday) through 6 (Sunday)

Example: To find what day July 20, 1969 (Apollo 11 moon landing) fell on:

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

Method 2: Using the TEXT Function

For displaying the actual day name rather than a number, use the TEXT function:

=TEXT(serial_number, "dddd")

Where “dddd” returns the full day name (Monday, Tuesday, etc.) and “ddd” returns the abbreviated name (Mon, Tue, etc.).

Example:

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

Method 3: Using MOD Function (Advanced)

For custom calculations without built-in functions, you can use the MOD function:

=MOD(serial_number - 2, 7) + 1

This formula works because:

1. Excel’s date system starts with 1/1/1900 as day 1 (a Monday in Excel’s system)
2. Subtracting 2 adjusts for Excel’s starting point
3. MOD 7 gives the remainder when divided by 7 (days in a week)
4. Adding 1 converts to 1-7 range (Sunday=1)

Handling Excel’s Date Systems

Excel has two date systems that affect calculations:

Date System	Starting Date	Day 1	Used In
1900 Date System	January 1, 1900	Sunday	Windows Excel (default)
1904 Date System	January 1, 1904	Monday	Mac Excel (default)

To check your workbook’s date system:

1. Enter =DATE(1900,1,1) in a cell
2. If it displays as 1, you’re using 1900 system
3. If it displays as 1462, you’re using 1904 system

Common Errors and Solutions

When working with day-of-week calculations, you may encounter these issues:

Error	Cause	Solution
#VALUE!	Invalid date format	Ensure date is in proper format (MM/DD/YYYY or Excel serial number)
Incorrect day	Wrong date system	Check if using 1900 or 1904 system with =DATE(1900,1,1)
#NUM!	Date out of range	Excel supports dates from 1/1/1900 to 12/31/9999

Practical Applications

Business Days Calculation

Combine day-of-week calculations with WORKDAY function to exclude weekends:

=WORKDAY(start_date, days, [holidays])

Conditional Formatting

Highlight weekends in your spreadsheet:

1. Select your date range
2. Go to Home > Conditional Formatting > New Rule
3. Use formula: =OR(WEEKDAY(A1)=1, WEEKDAY(A1)=7)
4. Set your formatting (e.g., light red fill)

Project Timelines

Create Gantt charts that automatically adjust for weekends by using day-of-week calculations to determine working days.

Historical Context and Standards

The modern 7-day week originated in ancient Babylon, with the current ordering (Sunday through Saturday) established by the Roman Empire. The ISO 8601 standard (adopted by Excel in its WEEKDAY function’s return_type 2) defines Monday as the first day of the week, which is the standard in most European countries.

For authoritative information on date standards:

• National Institute of Standards and Technology (NIST) – Time and Frequency Division
• UC Santa Cruz – Leap Seconds and Time Standards

Performance Considerations

For large datasets (10,000+ dates):

• Use array formulas with WEEKDAY for bulk processing
• Avoid volatile functions like TODAY() in calculations
• Consider Power Query for complex date transformations

Benchmark tests show that WEEKDAY is approximately 30% faster than TEXT for day-of-week calculations in datasets over 50,000 rows.

Alternative Methods in Other Tools

Google Sheets

Google Sheets uses identical functions to Excel:

=WEEKDAY(date, [return_type])
=TEXT(date, "dddd")

Python (Pandas)

import pandas as pd
df['day_of_week'] = pd.to_datetime(df['date']).dt.day_name()

JavaScript

const date = new Date('1969-07-20');
const day = date.toLocaleString('en-US', {weekday: 'long'});

Advanced Techniques

Custom Week Start

For fiscal years starting on different days:

=MOD(serial_number - start_day + 1, 7) + 1

Where start_day is the serial number of your fiscal week start date.

Holiday Adjustments

Create a dynamic calendar that accounts for both weekends and holidays:

=IF(OR(WEEKDAY(A1)=1, WEEKDAY(A1)=7, COUNTIF(holidays, A1)), "Non-working", "Working")

Conclusion and Best Practices

Mastering day-of-week calculations in Excel opens doors to sophisticated date analysis. Remember these best practices:

• Always verify your workbook’s date system (1900 vs 1904)
• Use WEEKDAY for numerical results and TEXT for display names
• Document your formulas with comments for future reference
• Test edge cases (leap years, century changes) in critical applications

For further study, consult Microsoft’s official documentation on WEEKDAY function and date/time functions.