Three Days and Three Nights

Objection: In Matthew 12:40, Jesus states He would be in the tomb three days and three nights. So why do most Christians believe He was crucified on a Friday, given that He rose again on Sunday? That doesn't add up to three days and three nights.

In the verse from Matthew, Jesus is simply using a Jewish idiom. As the footnote to this verse in the Archaeological Study Bible notes:

To count a part of a day as a whole day was a common Jewish reckoning of time.

Thus, by that reckoning, Jesus's words would be fulfilled simply by His being in the tomb part of three separate days, i.e., Friday afternoon, Saturday, and very early Sunday morning (or, to speak more strictly, any time past sunset on Saturday evening, since the Jewish day ran from sunset to sunset.) So there is no conflict once we recognize the idiomatic nature of the statement.

All cultures and languges, of course, have idiomatic expressions that need to be recognized as such and not taken in a strictly literal manner lest misunderstandings result (the classic example in English being "it's raining cats and dogs"). The same is true in the Bible. For instance, in the Old Testament, from what I understand, when God is described as "longsuffering", what the text literally says is that He is "long of nose". So long as we recognize the idiomatic nature of that expression there is no problem, of course. But if we were to insist on taking the idiom in a strictly literal manner then we would be left with the impression that God in the Old Testament is perhaps no more than an exalted human being like the Mormons believe, albeit one with a long nose! (While I suppose that is better than asserting God to be a dishonest wooden puppet like Pinocchio after being caught in a lie, it is nevertheless a grave error to be avoided all the same.)

Similarly, if we do not recognize the idiom in the passage from Matthew up above, we end up creating a Scripture contradiction that does not in fact exist otherwise.

After all, if we take Jesus' statement as referring to a literal 72-hour period, then there is a problem. Jesus died in the afternoon and was buried shortly before sunset, so under that scenario he would have to have been resurrected in the afternoon shortly before sunset, which is certainly not the impression one gets from the text. 

More importantly, the New Testament makes abundantly clear that Jesus was crucified the day before the Sabbath (cf. Mark 15:42), that is, Friday. Moreover, that this "sabbath" is to be understood in the normal sense, i.e., of the seventh day of the week, is shown by the fact that the "first day" of the week appears to have followed immediately after this sabbath (cf. Luke 23:54-24:1).

Also, we know that Christ was crucified on Friday, because constantly we see in the New Testament that Christ was raised on the third day (cf. Matt 16:21; 17:23; 20:19; 27:64; Mark 9:31; 10:34; Luke 9:22; 18:33; 24:7, 21, 46; Acts 10:40; 1 Cor 15:4).

And yet we also know that when the phrase "the third day" is used elsewhere in Scripture, it is used precisely in such a way that parallels the way Christians have always understood it when used in reference to Christ's resurrection. Thus, to give some examples:

And the LORD said unto Moses, Go unto the people, and sanctify them to day and to morrow, and let them wash their clothes, And be ready against the third day [...] (Ex 19:10-11a) 

But if the sacrifice of his offering be a vow, or a voluntary offering, it shall be eaten the same day that he offereth his sacrifice: and on the morrow also the remainder of it shall be eaten: But the remainder of the flesh of the sacrifice on the third day shall be burnt with fire. (Lev 7:16-17) 

It shall be eaten the same day ye offer it, and on the morrow: and if ought remain until the third day, it shall be burnt in the fire. (Lev 19:6) 

And Jonathan said unto David, [..] when I have sounded my father about to morrow any time, or the third day [...] (1 Sam 20:12) 

And he said unto them, Go ye, and tell that fox, Behold, I cast out devils, and I do cures to day and to morrow, and the third day I shall be perfected. (Luke 12:32)

From these parallels of how "the third day" is understood elsewhere in Scripture, we see that if Christ were raised on "the third day" (i.e., Sunday), then the "first day" (i.e, the day of the crucifixion) would be on Friday. Thus:

"To day" (the first day) = the day of the crucifixion, Friday
"To morrow" (the second day) = Saturday
"The third day" = Sunday morning (i.e., any time past sunset on Saturday, since, as stated above, the start of the next day started at sunset by Jewish reckoning.)

Thus, we see that the day which Christians have historically understood the crucifixion to have been on is shown to be in accordance with Scripture.