Knights always tell the truth.
Knaves never tell the truth any sentence uttered by a knave is false.
A mumbled an answer that the stranger could not understand. It will be a surprise, because you won't be able to figure out on which day it will occur until the class meets on the day of the test. It could happen on Monday, Tuesday, Wednesday, Thursday, or Friday, but I won't tell you which day. The class were clever. So we know the test can't be on Friday.