The problem with choosing at day at random is, what if it turns out to be Friday? Friday would not be a surprise, since the test will be either Monday, Wednesday or Friday, and so by Thursday the students would know by process of elimination that it had to be Friday.
The problem with choosing at day at random is, what if it turns out to be Friday? Friday would not be a surprise, since the test will be either Monday, Wednesday or Friday, and so by Thursday the students would know by process of elimination that it had to be Friday.