Example sentence:
When looking at the month view of an online calendar, users can toggle visibility of [whatchamacallit] days from adjacent months.
When you look at a monthly calendar, and the month begins on, say, a Wednesday, the dates of the preceding Sunday–Tuesday from the end of the the prior month are usually displayed (but grayed out). Same story at the end of the month. If you’re a designer of calendars, what do you call such days?