I do one day a week, sometimes two.
Monday is the standard one, after a heartier weekend and a few soda-pops it really does reset the system and you feel so much sharper so much quickly
I also try to do Thursday as well, but not religiously and sometimes just have a smaller intake as harder and I generally have eaten less on a Wed than a Sunday as well.
Not sure about the liquid thing though, water, black coffee and green tea AOTS