The reason is one of two things, maybe, both. The few extra days gives him additional employment credit for golden benefits from the state, retirement, dancing girls, a vacation home, what ever he can extort from the state. Second, of course, is that Friday is the day that the cash in unmarked envelopes is delivered for payoffs, skims, kick-backs, extortion, bribes and shares in crimes. There probably is an official payday for state employees between the two days in question, The Friday change allows him to get the piece of salary from those who owe him their jobs and from the other politicians who have phatom people on their payrolls. An honest media would have mentioned this.
Good point, You DON'T mess with the patronage system. ;-)