Earliest cultures believed the number three to be mystical, magic or representative of the supernatural. In their world, triads of natural forces aligned to create events that man could neither dispute, disrupt or control. These ancients were the first to notice that similar calamitous events seemed to arrive in groups of three; and that cycles of life also occurred in three distinct phases --such as past, present or future, or planting, growing and harvest.
From Vincent F. Hopper: "A single occurrence is of no significance. A repetition is noticeable but it might easily be the result of coincidence. A third occurrence, of the same nature, gives the event the impress of law." ("Medieval Number Symbolism: Its sources, Meanings…and Expression", p.5. Published 1938, New York.)
Over the centuries, the number three took on more significance than quantity or the mysterious. By the 18th century BC, the number three had become symbolic and had been adopted into the legal system.
From EW. Bollinger: "The number three (by Bible times) had come to represent that which is solid, real, substantial, complete and entire." ("Number in Scripture...and Spiritual Significance", p.95. Published 1921, London.)
From the Code of Hammurabi: "A man may...repudiate the purchase of a female slave after three days on approval.” (Law 278 translated; Babylon 1754 BC .)
From the King James Bible: When Jeroboam requested more lenient royal policies, Rehoboam chose to delay his answer and instead said: "Depart for three days, then return to me. So the people departed." (1 Kings 12:5. Rebellion of Israel against the House of David. About 970 BC)
From Roman Law: "Thirty days shall be allowed by law for payment of confessed debt and for settlement of matters adjudged in court. After this time the creditor shall have the right of laying hand on the debtor. The creditor shall hale the debtor into court...or they shall have the right to compromise. (If no) compromise the debtor...shall be brought to the praetor, into the meeting place, on three successive market days, and the amount for which they have been judged liable shall be declared publicly. Moreover, on the third market day, they shall suffer capital punishment or shall be delivered for sale abroad across the Tiber River." (From the "Twelve Tables". Table III – Execution of Judgement. 450 BC.)
From Jewish Law: "Once something is done three times it is considered a permanent thing. This concept is called a chazakah; which is defined as an act of property acquisition or the status of permanence that is established when an event repeats itself three times." (Detailed in the Talmud - 550 AD.)
The phrase three days and three nights is found in three places in the Bible: 1 Samuel 30:12, Jonah 1:17 and Matthew 12:40.
While there seems to be no argument that the event described in 1 Samuel (the illness of an Egyptian slave) did indeed take place during a literal three-day period, not so with the events described in Jonah and Matthew. In those two respects, there is considerable argument as to whether those events (Jonah in the big fish and Christ in the grave) lasted fully as long as stated or...if those two events even happened at all.
The Hebrew word for day is yom and the Greek word is hemara. Both words have similar meanings and can denote either : 1) the warm hours of the day, meaning the space of time from sunrise to sunset or 2) the entire space of time from one sunset to the next sunset.
The Jews in Bible reckoned one full day to be all the hours between two sunsets. The day began at one sunset and ended at the beginning of the next. It included both the evening and night darkness and the morning and day lightness. (See Gen. 1:4-5). However, these same Jews believed that any part of any day, whether the dark hours or the light hours, could also be considered a day.
From Rev. Dr. John Lightfoot: "…where many things are discussed by the Gemarists* concerning the computation of this space of three days, among other things these words occur:
1. "R. Ismael saith: 'Sometimes it contains four Onoth sometimes five, sometimes six." But how much is the space of an Onah?
2. R. Jochanan saith: 'either a day or a night.'
3. And so also the Jerusalem Talmud; R. Akiba fixed 'a day for an Onah, and a night for an Onah':
4. But the tradition is, that R. Eliezar Ben Azariah said: 'A day and a night make an Onah, and a part of an Onah is as the whole.'
5. And a little after, R. Ismael computeth: 'a part of the Onah for the whole.' ("Hebrew and Talmudical Exercitations of the Gospels…Corinthians, ", Vol 2, p. 210. Published 1658 – London.)
If, according to Jewish history and tradition, a day can contain both the light hours and the dark hours, then why was it important for the Bible writers to include the word night(s) when relating events that lasted three days. The addition of the word "nights" seems redundant.
From Jacobus deVoragine (Archbishop of Geno): “For right so as in the mouth of twain or of three is the witness established, right so in three days is proved all deed and fait veritable.” ("The Golden Legend or Lives of the Saints", Compiled in 1275 AD. 1st. Edition Published 1470 AD.)
Three days and three nights is a more precise phrase than simply citing "three days" (or simply citing "three nights"). It provides extra meaning along with the complete information.
In the introductory text, by using the specific phrase three days and three nights, the reader is meant to understand that not only did the event take place over a span of three days --parts or whole--, but that the event also met the criteria of that time period for being real, verified, complete, absolute and true.
In other words, Jonah survived three days, (parts or whole) in a large fish and the event was declared undeniably factual.