You can choose to back up your data Daily, Weekly or Monthly. If you choose Daily, the "What Day" box becomes grayed out. However, if you choose Weekly, you will need to select what day of the week, and if you choose Monthly, you will need to select what date of each month you would like the back up performed.
The last option is to choose a time. If you turn your computer off, then you will need to schedule the back up to run at some point while the computer is on. However, using the computer during the backup may make it impossible to back up some files, and the process of backing up will eat systems resources and make your system run slower.
If you leave your computer on 24/7, it makes more sense to schedule the backup while you are sleeping. If you set it for 2am or 3am, it will be late enough that it won't interfere if you happen to be up late, and early enough to make sure the backup is complete if you happen to get up early.