How do I get the first day of the week in Excel (with examples)?

Understanding Week Start Conventions in Excel

When working with time-based calculations in Excel, the definition of when a week begins is paramount. Different regions and industries adhere to different standards—some use Sunday (Return Type 1 in WEEKDAY function logic), while others mandate Monday (Return Type 2). The formulas we use here must account for this variability by manipulating the underlying date serialization numbers appropriately.

To derive the first day of the week for any given date, we rely on the intrinsic way Excel stores time: as serial Date values. Each date is represented by a sequential number counting the days since January 1, 1900. By determining the remainder of the division of the date serial number by 7—a task perfectly suited for the MOD function—we can calculate exactly how many days must be subtracted from the current date to reach the preceding Sunday or Monday.

The following two formulas represent the most efficient and mathematically sound methods for calculating the first day of the week, offering flexibility based on the required start day convention:

Formula 1: Calculating the First Day Assuming a Sunday Start

This formula is tailored for environments where the week officially begins on Sunday. It works by adjusting the serial number of the target date (A2) to ensure that the calculation correctly identifies Sunday as the zero-point in the 7-day cycle. Subtracting 1 from the date (A2-1) shifts the reference point, allowing the subsequent MOD function operation to return the number of days that have passed since the previous Sunday.

The result of the formula will be the serial date value corresponding to the most recent Sunday, inclusive of the current date if the current date is Sunday itself. This is a crucial distinction for accurate weekly reporting.

Formula 1: Get First Day of Week (Assuming First Day is Sunday)

=A2-MOD(A2-1, 7)

Formula 2: Calculating the First Day Assuming a Monday Start

If your reporting requirements dictate that the work week commences on Monday, you must use a slightly different adjustment factor. This formula shifts the reference point to account for Monday being the start of the week. By subtracting 2 from the date’s serial number (A2-2), we align the date’s weekday index such that the MOD function correctly identifies the offset required to reach Monday.

This calculation is particularly important in business and European contexts where the ISO standard for weekly reporting often mandates a Monday start. The resulting date will be the Monday corresponding to the start of the week in which the date A2 falls.

Formula 2: Get First Day of Week (Assuming First Day is Monday)

=A2-MOD(A2-2, 7) 

It is important to note that both of these powerful formulas assume that cell A2 contains the specific date for which you intend to find the corresponding first day of the week. You should adjust the cell reference (A2) to match your input data location.

Practical Implementation: Setting Up the Dataset

To illustrate the effectiveness and difference between these two methods, we will apply both formulas to a sample dataset containing various dates. This visual example will clearly demonstrate how the calculation changes based on whether Sunday or Monday is designated as the start of the week. Begin by creating a column in your Excel sheet, typically Column A, populated with the dates you wish to analyze.

For this demonstration, we use the following setup. Column A contains the input dates, while Columns C and D will house the calculated results for Sunday-start and Monday-start definitions, respectively. This organization makes comparison straightforward and enhances the clarity of the resulting output.

The following image shows the initial dataset structure we will use for our practical examples:

Step-by-Step Example Walkthrough

We begin the application process by entering the required formulas into the top row of our calculation columns. For the sample date located in cell A2, we will input the Sunday-start formula into cell C2 and the Monday-start formula into cell D2. This ensures that the calculation is tested correctly against the first data point.

The specific formulas to be entered are as follows:

• C2 (Sunday Start): =A2-MOD(A2-1,7)
• D2 (Monday Start): =A2-MOD(A2-2,7)

After successfully entering the formulas into cells C2 and D2, the next step involves leveraging Excel’s efficient fill handle functionality. Click on cell C2 and drag the formula down to cover the entire range of dates in Column A (up to C11). Repeat this process for cell D2, dragging the formula down to D11. This process instantly calculates the first day of the week for every date in your dataset, demonstrating the speed and scalability of this method.

Upon initial calculation, your spreadsheet should resemble the following visualization, showing the raw numeric output:

Understanding Excel’s Numeric Date Values

As illustrated in the preceding image, the default output of date arithmetic in Excel often presents results as generic numeric values rather than recognizable date formats. This is a standard feature of how the software handles dates internally, treating them as sequential integers.

It is crucial for accurate data interpretation to understand the meaning of these raw numerical results. Each number represents the total count of days that have elapsed since Excel’s predefined baseline starting point: January 1, 1900. For instance, the number 44563 signifies that 44,563 days have passed since 1/1/1900. These serial numbers are the engine behind all time-based calculations, including the MOD function used here, and reflect the true mathematical difference calculated by our formulas.

To make this data useful for human readability and analysis, the next logical step is to reformat these serial numbers back into standard calendar dates.

Converting Numeric Results to Standard Date Format

Converting the serial Date values back into a readable format is a simple process using Excel’s formatting tools. This step is essential for confirming the accuracy of the calculation and for presenting the data clearly in reports or visualizations.

To achieve this conversion, follow these steps:

1. Highlight the cell range containing the numeric results, which in our example is C2:D11.
2. Navigate to the Home tab located on the Excel ribbon.
3. Locate the Number Format dropdown menu (usually found in the ‘Number’ group).
4. Select the Short Date or Long Date option, depending on your presentation needs. The Short Date format (e.g., MM/DD/YYYY) is typically sufficient for this purpose.

The following image demonstrates the location of the necessary formatting tool within the ribbon interface:

Once the formatting is applied, the serial numbers instantly transform into recognizable dates, providing a clear view of the calculated first day of the week for every date in the input column. Your resulting spreadsheet should now look like the image below, with legible dates in columns C and D.

Verification and Interpretation of Results

With the formulas applied and the results properly formatted, we can now interpret the outcome based on our defined conventions. Column C consistently displays the first day of the week assuming a Sunday start, while Column D reflects the results assuming a Monday start. The difference between the two columns will typically be one day, reflecting the shift in the start boundary.

For instance, let us analyze the date 1/4/2022 (January 4, 2022), which falls on a Tuesday. We can use a visual calendar reference to verify the accuracy of our calculation:

Based on this calendar:

• If we consider Sunday to be the first day of the week, the week begins on 1/2/2022.
• If we consider Monday to be the first day of the week, the week begins on 1/3/2022.

These independently verified dates—1/2/2022 in Column C and 1/3/2022 in Column D—precisely match the output generated by our respective formulas. This confirmation proves the reliability and accuracy of using the subtraction and MOD function combination to dynamically calculate the start of any week based on flexible start conventions.

By mastering these efficient formulas, you gain a powerful tool for date analysis, eliminating the need for complex nested IF statements or reliance on potentially inconsistent built-in functions for boundary identification. These techniques ensure your date handling in Excel is robust, scalable, and tailored to meet diverse reporting standards.