Here it's in our supplement, states an employee's workweek will be 5 consecutive days...which would make Saturday optional.
Not necessarily optional. Just not part of your normal work week. The contract also designates contractual holidays (which by definition are not working days and often not part of the 5 consecutive days). But the contract also has wording that allows UPS to force drivers to work (not voluntary)on holidays;
To be clear, below is the wording addressing forcing on holidays, but it shows work outside the 5 days is not always voluntary:
Article 40;
"the Employer shall have the right to force part-time air drivers and then full-time air drivers to work starting in reverse order of seniority. If after exhausting the above steps scheduling needs are still not met, the Employer shall offer the work in seniority order within the package driver classification. If more drivers are still needed the reverse seniority order concept will be used for package drivers."
So since holiday work often falls outside the 5 consecutive days, yet can still be forced, your argument that all work outside the 5 consecutive days is voluntary is incorrect.
Honestly, they have been forcing for years on various Saturdays and holidays. If they are forcing, they have to play by the rules, but they can force a 6th punch in all but few locals that specifically address forcing a 6th punch. To say 6th punch is always voluntary misleads drivers into actions that will likely lead to discipline.