It is a simple system, so it does not matter.

Going to have to disagree here. It is a bit more complicated than that. However, if you only consider the effect of B checking everything on the same day vs not, then the simple system answer certainly applies. However, if you're flying multiple times per day to a particular destination and you happen to B check each of those planes on the same day, you get zero revenue for that B check day. If you spread out the B checks across the lower demand days (Weds,Sat,Sun, and then Tues/Thurs), then you'll capture far more seats sold than you would if you just took a day off.

A very rough estimate - let's suppose 4x daily 50 seat planes scheduled to a destination with an average demand of 175. Therefore, demand would look approximatly like this -

Mon - 200

Tues - 175

Wed - 150

Thurs - 175

Fri - 200

Sat - 150

Sun - 150

But wait. You're serving 200 seats per day every day. That means you'll have approx full loads Mon/Tues/Thurs/Fri and about 80% loads on Weds/Sat/Sun. Thus, you could cut Weds/Sat/Sun out with a minimal impact on your revenue, and then pick Tues or Thurs to drop as well, which means on the B check week, all your planes will be full compared to 80-90% full on a non-B check week.