Short answer: in the fridge, in an airtight tin — and they’re better after a night’s rest.
Once filled, macarons want to “mature”: a night in the fridge in an airtight container lets the filling soften the shells to that perfect chew, so they’re genuinely nicer on day two. Keep them chilled and eat within about four or five days. Bring them to room temperature for twenty minutes or so before serving — cold mutes the flavour. Unfilled shells keep a little longer, or freeze well; filled ones can be frozen too and thawed in the fridge.
Related:raspberry French macarons · glossary:ganache.