Here's how to use data validation to prevent data entry on weekends in Excel:

1. Select the cell(s): Choose the cell(s) where you want to restrict weekend entries.

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

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

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

```excel
=WEEKDAY(A1)>=6
```

Note: Replace "A1" with the actual cell reference you selected in step 1.

5. Explanation of the Formula:
- `WEEKDAY(A1)`: This part of the formula uses the WEEKDAY function to determine the day of the week for the entered date in cell A1 (or your chosen cell).
- `>=6`: This checks if the weekday number is greater than or equal to 6. In Excel, weekdays are numbered with 1 being Sunday and 7 being Saturday. So, this condition essentially checks for Saturdays and Sundays.

6. Error Message (Optional): In the "Input Message" box, you can type a message that will appear when users hover over the cell. This could explain why the data entry might be rejected.

7. Error Alert (Optional): In the "Show error alert" box, you can choose what happens when users try to enter a weekend date. You can display a "Stop" alert with a custom error message or an "Informative" alert explaining the restriction.

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

With this setup, whenever someone tries to enter a date that falls on a Saturday or Sunday, Excel will trigger the data validation rule and prevent the entry. You can customize the formula based on your specific needs, such as using a different cell reference or including holidays.