One house, built around a courtyard, and a January cold that improves it considerably.
Closed Tuesdays. The courtyard is worth arriving for.
Go in January. The courtyard is glassed and planted and warm while the city outside is doing its worst, and the contrast is half the experience. I have never found it crowded in winter, which cannot be said of April.
Nothing here is labelled in the way you expect, and that is deliberate — the arrangement is the exhibit. Resist the urge to look everything up until afterwards.
Boston is three and a half hours from New York on the train, which makes this page and the next one a single week rather than two trips. Do them in that order; the Gardner sets the ear for everything after it.