You asked "how does task manager [scheduler] know when it's a new Friday?" - it doesn't. You tell the task scheduler to run a particular program on a regular basis (for example, EVERY Friday, starting at a specific time). Once scheduled, it doesn't know it's a NEW Friday, it just knows it's Friday (and runs the task you've scheduled for a Friday). That's all I'm going to say about it in this thread (especially since I don't use the Task Scheduler that frequently).
Also, since discussing the details of the Task Scheduler is technically a Windows question, and not an ImageWin question, you may want to go to one of the various Windows forums (https://www.sevenforums.com/
are both good, depending on which version of Windows you are running) and ask for help there. If you have scheduling questions specific to ImageWin, then I suggest you start a new thread, since that discussion isn't necessarily related to your questions about Differential and/or Incremental backups.