building a timesheet.

Is that mm/dd/yyyy or dd/mm/yyyy? Either way, it's not a Wednesday...

Anyway, you can do it by creating two fields. Let's say the are called Date1 and Day1. As the custom calculation script of the latter enter the following code:

var s1 = this.getField("Date1").valueAsString;
if (s1=="") event.value = "";
else {
	var d1 = util.scand("dd/mm/yyyy", s1);
	event.value = util.printd("dddd", d1);
}

Make sure to adjust the first date format string to match the one you're using in Date1, of course...