Month
Tapping any day selects its whole month (first..last day) as a range, clamped to the engine bounds. The click is rejected with SelectionEvent.Intercepted when any day in it is disabled. Tapping a day inside the selected month clears the selection when allowClear is true.
Programmatic writes (SelectionEngine.replace, CalendarSelectionState.set and initialSelection) are snapped to the whole month containing Selection.rangeStart (clamped to the bounds), then validated like a tap; Selection.rangeEnd of the proposal is ignored.