Ingress
The moment a moving body crosses into a new zodiac sign. The Moon makes one every two or three days; the Sun makes twelve a year, and its Aries ingress opens the astrological year.
An ingress is a border crossing: a planet's ecliptic longitude ticks over from one sign into the next, at a moment that can be computed to the minute. The planet keeps its agenda and changes its costume, since the new sign restyles how it operates.
Different bodies cross at very different rhythms. The Moon changes sign every two to three days, setting the household weather; the Sun's twelve ingresses mark the seasons, the cardinal ones landing on the equinoxes and solstices; an outer planet's ingress is news, opening a chapter that can run for years.
Two bookkeeping notes. A Moon ingress always ends any void-of-course period. And in whole sign houses an ingress is also a house change, which is part of that system's appeal.