WeekDayName

I am trying to get the name of the day of the week on a form which already holds the date. I can get the number of the day within the week using the function WeekDay, and on the result of this I want to apply the function WeekDayName. This works in VB, but not in the Expression builder. I cannot use VB because I have a continuous form and so get the same day name for all dates, i.e. the name of the day corresponding to the last date focussed.
I need a way to apply WeekDayName in the expressionbuilder or a way to write VB for one detail only of a continuous form.