If you pre-enter lessons for the future (all of which are dated) then you have an unexpected day off for illness or whatever, there is a built in "Date Bump" feature that takes care of this. Simply go to the first assignment date that needs to move forward and double-click on the date. A window appears that allows you to move all lessons or just one subject for all students or just one student. Moved lessons will only land on school days according to your school calendar. You cannot bump lessons back in time - only forward. If you have to move them back in time, the dates can be manually changed.