Confirming the window is real
A site that describes itself as closed on Sunday is not always empty. Maintenance crews, a skeleton dispatch operation, security patrols and contractors all use the day, and any of them can occupy the areas the schedule was relying on. Establishing what actually happens on a Sunday, rather than what the roster suggests, is worth doing before the whole arrangement depends on it.
Access is the other question. Weekend access frequently differs from weekday arrangements, with different alarm codes, a different gate procedure and often nobody on site to resolve a problem. A schedule that puts its most substantial work into a window with no support arrangement is exposed, and that should be settled during mobilisation rather than on the first Sunday.
