Here's how to use data validation to prevent data entry on specific days of the week in Excel:

1. Select the cells: Choose the cell(s) where you want to restrict data entry based on weekdays. You can select a single cell or a range of cells.

2. Go to Data Validation: Navigate to the "Data" tab on the Excel ribbon. In the "Data Tools" group, click "Data Validation."

3. Set Allow to Custom: In the "Settings" tab of the Data Validation window, under "Allow," choose "Custom" from the dropdown menu.

4. Create the formula: In the "Formula" box, enter the following formula:

```excel
=WEEKDAY(A1,2)<>6
```

Explanation of the formula:

- `WEEKDAY(A1,2)`: This part uses the WEEKDAY function. "A1" is the cell reference you want to validate (replace A1 with your actual cell if different). The `2` argument specifies a Monday-based week numbering system (1 for Sunday-based).
- `<>6`: This checks if the result of the WEEKDAY function (day of the week number) is not equal to 6 (Saturday).

5. Error message (Optional): You can customize the error message that appears when someone tries to enter a date on a weekend. In the "Input Message" box, type a clear message explaining the restriction (e.g., "Weekends not allowed").

6. Error title (Optional): Similarly, you can set an error title in the "Show error message if invalid value is entered" box (e.g., "Invalid Date Entry").

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

With this setup, Excel will only allow data entry in the selected cells if the corresponding date falls on a weekday (Monday to Friday). Any attempt to enter a date on a Saturday or Sunday will trigger the custom error message.