Here's how to use data validation to restrict data entry to specific weekdays in Excel:

1. Select the cells: Choose the cell(s) where you want to allow only weekday entries.

2. Data Validation: Go to the "Data" tab and click "Data Validation" in the "Data Tools" group.

3. Settings Tab: In the "Settings" tab of the Data Validation window, choose "Custom" under "Allow."

4. Formula for Weekday Check: Enter the following formula in the "Formula" box:

`=WEEKDAY(A1,2)<6` (Replace A1 with your actual cell reference)

Explanation of the Formula:

- `WEEKDAY(A1,2)`: This part uses the WEEKDAY function. It takes the cell reference (A1 in this example) and a return type argument (2). With a return type of 2, WEEKDAY considers Monday as 1 and Sunday as 7.
- `<6`: This compares the weekday number returned by WEEKDAY to 6.
- If the result is less than 6 (meaning Monday to Friday), the formula evaluates to TRUE, allowing the data entry.
- If the result is 6 or 7 (Saturday or Sunday), the formula evaluates to FALSE, restricting data entry.

5. Error Message (Optional): You can set an informative error message in the "Input Message" box that appears when users hover over the cell. For example: "Weekdays only (Mon-Fri)".

6. Title (Optional): You can also set a title for the data validation in the "Show error message when cell is invalid" checkbox area.

7. Click OK: Click "OK" to close the Data Validation window.

Now, when users try to enter a date that falls on a weekend (Saturday or Sunday), they will receive an error message and won't be able to enter the data. The cell will only accept dates corresponding to weekdays (Monday to Friday).