a good time to start a diet (things which must decrease), a waxing Moon a good time to start a bank account or IRA (things which must increase)

The phase of the Moon from Full to New, when it gradually becomes less visible

The moon between full and new when its visible part is decreasing. See phases of the moon.

The Moon from Full Moon to New Moon; the Moon from the opposition to the Sun until the conjunction.

Waning Moon is a solitaire card game played with two decks of playing cards. The name comes from the tableau that resembles a crescent moon and the fact that it "wanes" as more cards are played in the foundation.