Norman Geisler, When Critics Ask: Most biblical scholars believe that Jesus was crucified on Friday. They take the phrase "three days and nights" to be a Hebrew figure of speech referring to any part of three days and nights. (p. 343)

Matthew Henry’s Commentary on the Bible: Now this sign of the prophet Jonas he further explains here; (v. 40) As Jonas was three days and three nights in the whale’s belly, and then came out again safe and well, thus Christ shall be so long in the grave, and then shall rise again.… He continued in the grave just as long as Jonah continued in the fish’s belly, three days and three nights; not three whole days and nights: it is probable, Jonah did not lie so long in the whale’s belly, but part of three natural days (nychthemerai, the Greeks called them); he was buried in the afternoon of the sixth day of the week, and rose again in the morning of the first day; it is a manner of speech very usual; see 1 Ki. 20:29; Esth. 4:16; 5:1; Lu. 2:21. (computer file)

The New John Gill’s Exposition of the Entire Bible: So shall the Son of man be three days and three nights in the heart of the earth.

…it was on the sixth day of the week, we commonly call "Friday", towards the close, on the day of the preparation for the sabbath, and when the sabbath drew on, that the body of Christ was laid in the sepulchre; where it lay all the next day, which was the sabbath of the Jews, and what we commonly call "Saturday"; and early on the first of the week, usually called "Sunday", or the Lord’s day, he rose from the dead; so that he was but one whole day, and part of two, in the grave. To solve this difficulty, and set the matter in a clear light, let it be observed, that the three days and three nights, mean three natural days, consisting of day and night, or twenty four hours, and are what the Greeks call "night days"; but the Jews have no other way of expressing them, but as here; and with them it is a well known rule, and used on all occasions, as in the computation of their feasts and times of mourning, in the observance of the passover, circumcision, and divers purifications, that "a part of a day is as the whole": and so, whatever was done before sun setting, or after, if but an hour, or ever so small a time, before or after it, it was reckoned as the whole preceding, or following day; and whether this was in the night part, or day part of the night day, or natural day, it mattered not, it was accounted as the whole night day: by this rule, the case here is easily adjusted; Christ was laid in the grave towards the close of the sixth day, a little before sun setting, and this being a part of the night day preceding, is reckoned as the whole; he continued there the whole night day following, being the seventh day; and rose again early on the first day, which being after sun setting, though it might be even before sun rising, yet being a part of the night day following, is to be esteemed as the whole; and thus the son of man was to be, and was three days and three nights in the grave; and which was very easy to be understood by the Jews;… (The New John Gill’s Exposition of the Entire Bible, Modernized and adapted for the computer by Larry Pierce of Online Bible. All Rights Reserved, Larry Pierce, Winterbourne, Ontario.)

Jameison-Faussett-Brown Commentary: The period during which He was to lie in the grave is here expressed in round numbers, according to the Jewish way of speaking, which was to regard any part of a day, however small, included within a period of days, as a full day. (See 1 Samuel 30:12, 13; Esther 4:16; 5:1; Matthew 27:63, 64, etc.) (computer file)

Josh McDowell, Answers to Tough Questions Skeptics Ask About the Christian Faith: That the expression "one day and one night" was an idiom employed by the Jews for indicating a day, even when only a part of a day was indicated, can be seen also in the Old Testament.

For example, I Samuel 30:12, 13 (KJV), "For he had not eaten bread or drunk water for three days and three nights," and in the next verse, "My master left me behind… three days ago."

Just as clearly, Genesis 42:17 shows this idiomatic usage. Joseph imprisoned his brothers for three days; in verse 18, he speaks to them and releases them, all on the third day. (Computer file)

Gleason Archer, Encyclopedia of Bible Difficulties: According to ancient parlance, then, when you wished to refer to three separate twenty-four-hour days, you said, "Three days and three nights"—even though only a portion of the first and third days might be involved. (p. 328)

Albert Barnes, Barnes Notes on the New Testament: It will be seen, in the account of the resurrection of Christ, that he was in the grave but two nights and a part of three days. This computation is, however, strictly in accordance with the Jewish mode of reckoning. If it had not been, the Jews would have understood it, and would have charged our Saviour as being a false prophet; for it was well known to them that he had spoken this prophecy. Such a charge, however, was never made; and it is plain, therefore, that what was meant by the prediction was accomplished. (Kregel Publications, Grand Rapids, MI, 1966, p. 60)