Temporary extended hours

What would be the easiest way to make time slots available outside of regular office hours, but only for one day?  For example, if the "Business Hours" are normally set to close at 5:00pm on Monday, but I have one particular Monday where I want to be available until 8:00pm, what's the easiest way to apply that without inadvertently making available slots after 5:00 on other Mondays?

Setting "time off" is really easy; I can't find a simple solution to "extended hours."