Custom Date format Excel 365 ddd/mm/yy

Unfortunately, Excel for Web currently does not support creating or editing custom number/date formats (as far as I know).
You can only select from the pre-defined list of formats under “Number Format → Date.”

This is a known limitation — even though the web app preserves existing custom formats created in the desktop app.

So if you try to type ddd - dd mmm - yy (or your desired Mon - 01 Jan - 25) directly in the web UI, it won’t accept it.

Workarounds:

Use a helper column with a TEXT formula

If you want to stay fully within Excel for Web, use a formula instead:

=TEXT(A2, "ddd - dd mmm - yy")

Then your output will look exactly the same, e.g.:

Mon - 01 Jan - 25

You can even hide the raw date column and just display this formatted version in your to-do list.

This is dynamic — if you change the date in A2, the formatted result updates automatically.

Use a helper column with a TEXT formula

If you want to stay fully within Excel for Web, use a formula instead:

=TEXT(A2, "ddd - dd mmm - yy")

Then your output will look exactly the same, e.g.:

Mon - 01 Jan - 25

You can even hide the raw date column and just display this formatted version in your to-do list.

This is dynamic — if you change the date in A2, the formatted result updates automatically.

This is a suggestion.

My answers are voluntary and without guarantee!

Hope this will help you.