Time out
Keep in mind that the CLX doesn't calculate the day-of-the-week (DOW), it only keeps track of the current day.
So your logic is going to depend on whether or not the PLC is ON when you see a transition change in days.
For example, say that you compare the Current_Day to the Stored_Day. If they aren't equal, you'll increment the DOW counter, and move Current_Day to Stored_Day. Standard, straightforward logic.
But if they shut the plant down on Christmas eve (Sat) including powering off the PLC panel, leave it off all day Christmas (Sun), and power the plant back up on the 26th(Mon), your PLC will notice that the date is different, and will increment the day by one. The PLC will think it's Sunday, when in reality it's Monday. Ouch.
It is possible to calculate the DOW from the date. There are a couple of methods. One is just to calculate the number of days since 1/1/2000 (a Saturday, I think), and do use a MOD instruction (if the CLX has one - I don't remember and don't have the software here at home.
On the other hand, you can add something to your Operator Interface that tells the user what DOW the PLC thinks it is, and allows them to adjust it. Then the Standard, straightforward logic is sufficient.