Inspect the master calendar (Options, Calendars) and determine whether or not you need to customize the workweek (Workweek button) or the holidays (Holiday button). Changes made to the workweek or holidays will automatically affect the appropriate days in every month of the Master Calendar. If you'd rather not make adjustments to the Master Calendar at this time, that's fine, but keep in mind that 1) the Master Calendar determines which dates your tasks are allowed to occur on and 2) whenever you create a resource in the schedule, the resource's calendar that Time Line automatically creates for you will look just like the Master Calendar.