Calendar Confusion Puzzle

If I said that in three days' time it would be a Thursday, I am sure that most of you would have no difficulty telling me that today was a Monday.

Try this one then. Yesterday was two days before Monday. What day is it today? Yes, you're right again. It's Sunday. Do you get the idea?

Now let's tackle a similar question from The National Mathematics Contest (1991) Paper:

Three days ago, yesterday was the day before Sunday. What day will it be tomorrow?