Just starting to try and use the Calendar feature. It was wondering if it's possible to input 'Rest Days' into the calendar in the same sort of way as events/races/workouts?
Sure. You can just create events titled ‘Rest Day’ for the days in question. Or you can create a custom workout (of type Other, perhaps) with just a single Recover step, so that you can drag-and-drop it reusably onto the Calendar.