To automatically capitalize day names in Excel, follow these steps:

1. Open Microsoft Excel.
2. Click on the File tab.
3. Click Options.
4. Click Proofing.
5. Click AutoCorrect Options.
6. Check the box next to Capitalize day and month names.
7. Click OK.

Now, whenever you type a day or month name into a cell, it will automatically be capitalized. For example, if you type "january" into a cell, it will be changed to "January".