E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
properly means the ability to judge. Hippocrats said that all diseases had their periods, when the humours of the body ebbed and flowed like the tide of the sea. These tidal days he called critical days, and the tide itself a crisis, because it was on these days the physician could determine whether the disorder was taking a good or a bad turn. The seventh and all its multiples were critical days of a favourable character. (Greek, krino, to judge or determine.)