Hi Jule,
Here is a similar post outlining how to take out the time in a DateTime field. To insert the day of the week, I suggest dragging a textbox object onto your screen to show the value. From here, you can use a Date Function action (specifically, the Get Day of Week function) and then an Enter Value into Field on Screen action to place it in your new textbox.
There are multiple ways to customize this, but if you're just looking to view the day of the week on the screen, this may be the simplest solution. Let me know if you get stuck.
Jason