How to define a date Custom Format to show first 2 letters of the day's name

To show the letters requires ddd in the format string, and there are no functions allowed in custom formats, so the only way would be to use a formula in another cell, like

=LEFT(TEXT(A2,"ddd"),2) & ". " & TEXT(A2, "dd mmm yyyy")

One thing I would add is that Bernie's solution converts your date serial number to a text string. I agree with him that there is not a custom format code for "two character abbreviation of day of the week" (you can check for yourself here: https://support.microsoft.com/en-us/...rs=en-us&ad=us ). The decision for you as a programmer is deciding between showing the date in a specific format as a text string and storing the date as a date/time serial number. I feel like there are a lot more problems on this forum related to storing numbers as text than I see problems solved by storing a number as text. It will be up to you as the programmer to decide if, in your specific case, storing the number as text will really work for you.