Planetary joy
Planetary joy is the house a planet is said to rejoice in, an early Hellenistic scheme giving each of the seven a place where its nature is most at home.
Mercury joys in the first, the Moon in the third, Venus in the fifth, Mars in the sixth, the Sun in the ninth, Jupiter in the eleventh and Saturn in the twelfth. The pattern is not decorative: the benefics take the houses of good fortune and the malefics the houses of difficulty, above and below the horizon respectively.
The scheme is older than the house meanings we use, and several of those meanings appear to derive from it rather than the other way round, which is why the sixth reads as illness and the twelfth as confinement.
Frequently asked
Is a joy the same as a dignity?
No. Dignity is a relationship to a sign; a joy is a relationship to a house, and the two are counted separately.
Does a planet in its joy get stronger?
Tradition treats it as more comfortable and more itself rather than as stronger in the way essential dignity means strong.