NYCHTHEMERON, amongst the ancients, signified the whole natural day, or day and night, consisting of twenty-four hours, or twenty-four equal parts. This way of considering the day was particularly adopted by the Jews, and seems to have owed its origin to the expression of Moses in the first chapter of Genesis, "The evening and the morning were the first day." Before the Jews had introduced the Greek language into their discourse, they were accustomed to indicate this space of time by the simple expression of a night and a day.
It is proper here to observe, that in all the eastern countries any part of a day of twenty-four hours was reckoned for a whole day; and that a thing which was done on the third or seventh day from that last mentioned, was said to be done after three or seven days. The Hebrews, having no word which exactly answered to the Greek nycthemeron, signifying a natural day of twenty-four hours, used night and day, or day and night, instead of it; so that to say a thing happened after three days and three nights, was, with them, the same as to say that it happened after three days, or on the third day. This being remembered, will explain what is meant by "the Son of Man's being three days and three nights in the heart of the earth."