Friday afternoon is the time we run DI7 - just before the weekend break. It doesn't really matter when you do it - just choose a quiet time. It doesn't take long to do so you don't waste much valuable computer time and remember, your backup could be a lifesaver.
Before running we remove all useless files and trash, including .tmp files, internet rubbish etc. and empty the recycle bin. There is no necessity to do this but there isn't much point in saving or backing up rubbish.
Remember you ought to do a daily backup of files and settings in addition to a weekly full system backup. Also, back up before you make any big changes to your system - hardware or software.