foo must be Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, or Sunday

Message

foo must be Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, or Sunday

Explanation

day elements must be one of the following strings: "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", or "Sunday".

Solution

Fix or remove any day elements that are invalid.

Not clear? Disagree?

You might be able to find help in one of these fine resources.