It means exactly what it says. If you read it in a book or paper, the context should help you out.

Sunday is a day of rest. So is Saturday. Monday-Friday are work days. By saying "Every day is not Sunday", it simply means that not all the other days (like M-F) are going to be easy like Sunday.

oh I see !

In my country if you tell to someone "every day is not Sunday", this is means if you are lucky don't forget that your luck can turn any time...