B-check on Saturday, followed by Sunday/Wednesday/Tuesday/Thursday--never on Monday/Friday because these are the biggest travel days. The exception being if you fly a route 7x on 7 different aircraft and you can do one on each day of the week so you have the same number of seats flying everyday.
If you B-check on Monday/Friday, you are hurting yourself. These are the days where you have the most demand/traffic versus the weekend/middle of the week. It may not make a big difference if you have no competition, but if you're flying against someone and you are fighting over 500 seats on Monday and only 350 seats on Saturday, you should be hitting Monday with as much frequency as possible and B-checking on Saturday.