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

1. Select the cells: Choose the cell(s) where you want to allow only date entries. You can select a single cell or a range of cells.

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

3. Settings tab: In the Data Validation window, ensure you're on the "Settings" tab.

4. Allow: Under "Allow," choose "Date" from the dropdown menu. This tells Excel you only want to allow date entries in these cells.

5. Data: Now, you can define how to restrict the acceptable dates:
- Between: Select "between" to allow dates within a specific range. Enter the start and end dates in the "Start date" and "End date" boxes. These can be actual dates you type in (e.g., "01/01/2024") or formulas referring to other cells containing dates.
- Greater than: Choose "greater than" to allow dates after a specific date. Enter the minimum allowed date in the "Data" box.
- Less than: Select "less than" to allow dates before a specific date. Enter the maximum allowed date in the "Data" box.
- Equal to: Choose "equal to" to only allow entries matching a specific date. Enter the exact date in the "Data" box.

6. Input Message (Optional): In the "Input Message" tab, you can type a message that will appear when users select the cell. This can explain the allowed date format or range.

7. Error Alert (Optional): In the "Error Alert" tab, you can choose to display a custom error message if users enter an invalid date.

8. Click OK: Once you've defined your settings and any optional messages, click "OK" to activate data validation for the selected cells.

Now, when users try to enter data in these cells, Excel will only accept valid dates based on your chosen criteria. If they enter an invalid date, they will either see your custom error message or be prevented from entering anything depending on your settings.