How can I define weeks?

If your school has different timetables on odd and even weeks (sometimes called week "A" and week "B"), you can simply define that you have two or more weeks in this way:

Then for each lesson, you can specify in which week it can be. Default option for new lessons is "All weeks". This means, that this lesson will be at the same positions in all weeks. If you change this to lets say Any week, the software will allow you to place this lesson into week 1 OR 2.