You can't. I suggest changing the number format to accomplish this instead:
1. Select the desired cells
2. Select Format Cells Number tab
3. Select Custom from the Category list
4. Enter "dddd / mmmm d" in the Type window
5. Then just enter the date in the cells in any recognized date format.
Never mind entering the day.

Alternatively, forget the "/". Use the <Alt key to force a new line within
the same cell:
1. Type in the day (e.g. "Sunday")
2. Hold down the <Alt key and press Enter
3. Type the date

Regards,
Greg

"Cindy" wrote:

I would like to know how to divide a cell into diagonally. I want to be able
to type the day of the week for one part, then have the line / and then the
date. Any ideas? Monday/April 3 for example.