FT employees will be required to work as needed. PT employees can NOT be forced to work on what should be their normal day off. It is understood that PT employees may have other requirements that prevent them from altering their normal schedule.
During peak, Seniors and OPS managers may place all PT employees on the schedule so that it appears they have to work or it will count as an absence. However, if you are PT, then you can take your regularly scheduled time off. If you are scheduled and unable to make it in, then I would talk to the scheduling manager or the Senior to let them know that your life is based on having Mondays off and you are unable to alter your schedule. Since the schedule is usually posted by 1200 on the Wednesday prior I would let them know tomorrow or by Tuesday at the very latest. If there are any issues I would contact HR and possibly the MD.
The reaction of co-workers might be mixed though. Everybody is just trying to get through peak and wants to get home Xmas eve to be with their families. Some might applaud PT employees for taking a stand, but others might be resentful because it means more work for them. If you can make it but need to be off by a certain time, then I would let the scheduling manager know that too. That way you are helping out the other couriers but made it clear that you need to be off and can not take additional stops.
I want to repeat that PT can not be forced to change their shifts to accommodate FedEx, not even at peak.