Hexagram 24 is one broken line at the bottom of five unbroken ones, the first yang returning after everything has gone dark, and its judgement contains one of the book's most concrete and most argued-over phrases: on the seventh day comes return.
The plain reading is a cycle of seven units, and the traditional application is calendrical. In the Han-period scheme of sovereign or waxing-and-waning hexagrams, this figure is assigned to the winter solstice: the moment the light begins to come back, marked at the exact point when there is least of it. That assignment is a Han systematisation and not part of the core text, but it fits the figure so well that it has never been dislodged.
What the seven days actually count is disputed. Proposals include seven months, seven stages of a cycle, and the six lines of a hexagram plus one to begin again. The text does not specify. Wilhelm gives the phrase as it stands and lets it be; Legge discusses the difficulty; later translators tend to note it as unresolved.
The judgement also says that friends come without blame, that going out and coming in are without error, and that it furthers one to have somewhere to go. For a hexagram about a turning point at the bottom of a decline, that is a notably practical set of assurances: movement is safe again.
The figure is also the standard illustration of a principle running through the whole book, which is that reversals are located at extremes rather than in the middle. The light returns at the point of least light, not at the point where it would be convenient. Nothing in the sequence turns gradually.