It is a little odd, and took me a while to figure it out.
So first, you must convert the weekday(s) into a number. Sunday is 0, Monday is 1, Tuesday is 2, etc, Saturday is 6, yeah.
Next, we create a conditional switch and put this in it as a script:
pbIsWeekday((variable number),(a comma separated list of weekday numbers)). So, an NPC that appears on Friday only and does not store the day would be
pbIsWeekday(-1,5), while an event that only takes place on Mondays and Wednesdays and stores the day in variable 1 would be
pbIsWeekday(1,1,3). But if you want an event that appears everyday but Mondays and Wednesdays, we would invert it, so
!pbIsWeekday(1,1,3).
If you want an in-game example, open Map id 29, the Natural Park Entrance, and look at the guard.
Wiki on Timed Events and setting them up.