Leave is usually Sat-Fri with either three wrap days off before a block or four wrap days off after a block. You get one of each type per leave season (winter and summer). If the leave block plus wraps is entirely within a month, you can slide the start day +/- 3 days when bidding for work. You can either bid for two weeks together or two separate weeks. Awarding of leave is done on a points system, not seniority.
Duty free weeks are Sun-Sat and have a non-assignable day off before them. I think the winter DFW is 8 days now.
You can choose to work in duty free weeks or wrap days but not leave (unless the company is desperate!)