Go to this page and learn the Doomsday Rule for finding the day of the week. After some practice you will amaze your friends. This rule does not solve all calendar-related problems. In many instances, days are organized into two-week fortnights. For instance, the US Government pays its workers according to a two-week schedule.
Some companies operate according to a two-week plan.
There is a recycling company near where I live that picks up every two weeks. Since it may be important to know when you're paid, a rule is needed for fortnights. The Doomsday rule will tell you the day of the week but will not tell you which week of the fortnight it is. Before describing a Doomsday rule for fortnights, one must agree to a standard fortnight. In other words, the fortnight starts out with a left week and ends with a right week.