How to show weekday in Windows 10 clock in taskbar?

The weekday will be shown on your horizontal taskbar when you either

• Have Small Taskbar Icons enabled and the Taskbar is at least 3 rows high or
• Have Small Taskbar Icons disabled and the Tasbar is at least 2 rows high

If you use the vertical taskbar

• You need to have it at least 80 pixels wide @ 100% scaling

If you can't pull on your taskbar to resize it right-click it and uncheck Lock all taskbars

You can use https://github.com/White-Tiger/T-Clock for this, it provides many other options beside custom formats for the clock

• Go to: Control panel -> Clock and region -> Region
• in Region 'Format' tab go to 'Additional settings'
• in the date format box you can change short date. My format is : M/dd-ddd