Schedules can be challenging in general, let alone when working globally. For instance some European groups take 4 weeks of vacation off. For a SME whose input is critical to your project, that could be a significant impact.
While you cannot ask for an itinerary and hotel numbers, you can review the schedule with those that will be out of the office, week by week and milestone by milestone. This will help you prepare a Plan B and authorized actions in their absence if needed.