Hi Scott,
I don't think the weekday is part of the built in getTime function. We'll have to check with the dev team if it is.
Instead, you can define your own javascript function. There's two ways to do this - one is by registering your own custom function, and one is to use an IFrame. We'll go with the IFrame approach since this custom javascript is pretty limited in scope. And with the IFrame, you can set it and forget it. FYI, SmartBuilder calls an IFrame a "Web Object" - I use the terms interchangeably.
Check out the attached lesson and image - it should even work in Preview. You can copy the Wrb Object into your lesson, and the javascript inside should come with it (you can make it smaller and hide it offscreen - it still needs to Start Visible, though). Then create a similar action from the Web Object block to set your variable. Keep in mind that "onPageLoad" as a trigger might happen before the Web Object has a chance to load, so we're using "Web Object.onShow". The blue "get data" block comes from the Web Object's flyout panel.
That ought to do it! Let us know if you have any questions.