[wartijn_]

I think the "Let’s See if it actually rains more on the weekend." is not really want the author actually wants to test. The text in the image above says "What do you call the day after two days of rain? Monday". This suggests that Saturday and Sunday are the rainiest and second rainiest days of the week.

So what the author wants to see is if both Saturday and Sunday are rainier than any other week. So the problem isn't that you can't differentiate between those two days, but that you can't between either of those days individually and Monday.