In fact, they did. The Greek words
paraskeue (preparation day) and
prosabbaton (day before the sabbath) are the everyday words for Friday. So
all the Gospel authors said it was the sixth day.
From
A Greek-English Lexicon of the New Testament and Other Early Christian Literature by Bauer, Danker, Arndt, and Gingrich (BDAG), the standard lexicon for biblical Greek:
it was Friday, on which day everything had to be prepared for the Sabbath, when no work was permitted Mt. 27:62. . . . For Christians as well [paraskeue] served to designate the sixth day of the week . . . and so in Mod. Gk. For Christians it is a fast day, as the day of Jesus' death [Didache 8:1].
Here's the passage referenced from the Didache, a late first- or early second-century treatise on Christian teaching and practice:
And let not your fastings be with the hypocrites, for they fast on the second and the fifth day of the week; but do ye keep your fast on the fourth and on the preparation (the sixth) day [Gr.
paraskeuen]. (Tr. J. B. Lightfoot;
source.)
All of the Gospel writers use this word to denote the day of the crucifixion (Matt. 27:62; Mark 15:42; Luke 23:54; John 19:14,31,42). Mark adds one extra bit of information, however, and explains the word further: "it was the day of Preparation, that is, the day before the Sabbath." In Greek, "the day before the Sabbath" is a single word:
prosabbaton. Again, BDAG have this to say: "i.e.
Friday, used to explain the word [
paraskeue]
Mk 15:42."
So all four Gospel authors do in fact tell us, explicitly, that the day Jesus was crucified was the sixth day of the week.
Have you noticed that of all the Gospels, only Matthew uses that "three days and three nights" idiom? If it's so important that Jesus was dead and buried for three literal days and nights, no less than 72 hours, then why don't the other Gospels use that phrase as well?
It's idiomatic language. Matthew is writing to a Jewish audience, showing that Jesus is the Messiah promised to them by their prophets. The other Gospels (as well as Matthew) use more direct phrasing such as "he will be raised on the third day" (Matt. 20:19), "on the third day he will rise" (Luke 18:33), or "after three days he will rise" (Mark 9:31;10:34)--probably to avoid confusing Gentile readers who might not be familiar with Jewish idioms.
The
first day is the day the event happens. The second day is the day after, and the
third day is the day after that. See, for example, Luke 13:32: "Go and tell that fox, 'Behold, I cast out demons and perform cures today and tomorrow, and the third day I finish my course.'" In Jesus' own way of speaking, if the "third day" is the day after tomorrow, then "today" must be the first day.
The risen Christ met two disciples on the road to Emmaus on the "very day" of the resurrection (Luke 24:13). They did not realize who he was, and in explaining what they knew, they told him, "it is now the third day since these things happened" (v. 21). If the women and Peter discovered the empty tomb "that very day," the first day of the week (i.e. Sunday), and that was the
third day since Jesus died, then the
second day was Saturday, the regular weekly Sabbath, and the
first day--the day the crucifixion happened--was
Friday.
Gosh. It's like all those Christians have been observing Good Friday all these centuries out of tradition, but they've actually had it right all along.
Fundamentalists latch on to the "three days and three nights" idiom, and miss the point. The sign of Jonah was that Christ, like Jonah, would be buried in the "heart of the earth." It's not the exact number of hours he would be there. The Bible explains itself quite adequately, if only you take the time to pay attention and actually read what it says.