Christ dies around the time of the 9^th hour of the preparation day.

Christ was placed securely in the tomb before the night, the start of this first day (of the sequence of 3 day and 3 nights).

on the 3^rd day …
Not on the 4^th day.

The 9^th hour of the preparation day. Reference:

Matthew 27:45-50, 62; Mark 15:33-39, 42; Luke 23:44-46, 54; John 19:14, 31, 42

Christ was placed on the stake at the 3^rd hour, Mark 15:25. From the 6^th hour until the 9^th there was darkness over the land, Mark 15:33.

About the 9^th hour Christ died.

The 9^th hour is equivalent to 3:00 in the afternoon. As the gospels relate, at the 9^th hour, after He cried out to the Father, Christ died. Yet His body remaining on the stake until the evening (Christ was dead awhile. cf. Mark 15:44-45).

Reference:

Mathew 27:57–60; Mark 15:42-46; Luke 23:50-54. John 19:30-42.

During the even, the 12th hour of the day, Christ was placed in the tomb.

The evening of a day commences at the 12^th hour of the day. Reference: Matthew 20:6-12.  Verse 6 read: the 11^th hour. Then verse 8 read even was come. The even would be the 12^th hour. Why? Christ said “are there not 12 hours in the day” (John 11:9). The 12^th hour is the last hour of the day and is called the evening.  When the 12^th hour (even) of the preparation day came, Joseph Arimathea asked for the body of Christ to put into the tomb.

After the evening (the completion of the 12^th hour) of the preparation day concluded, the night (also known as the 1^st hour of the night) of the next (new) day commenced.

That new day was the Sabbath (High Day) Matthew 27:62; Mark 15:42; Luke 23:54; John 19:31 and was also called the First Day of The Feast of Unleavened Bread, a High Day.

The first day of Unleavened Bread was the 1^st Day of the required 3 days and 3 nights in which Christ had to be in the tomb.

It was necessary and required that Christ be in the tomb 3 days and 3 days.

Notice: Matthew 12:39-40.

“But he answered and said unto them, An evil and adulterous generation seeketh after a sign: and there shall be no sign given to it but the sign of Jonas the prophet: For as Jonas was three days and three nights in the whale’s belly; so shall the Son of Man be three days and three nights in the heart of the earth.”

Three days and three nights equates to 72 hours.

Elements to consider about 72 hours in the tomb:

Did Christ say he would be in the heart of the earth 72 hours?

No! And the reason is obvious. The hour (60 minutes) which we utilize today was not yet in practice then. The sundial was operational then and helped mark off time. One could know the 3^rd hour, the 9^th hour, the 11^th hour, etc by the positioning of the sun (in the sky and on the sundial). Within each hour degrees of that hour were marked off. But for that generation, precision of minutes was not an exacting practice.

We do know that Christ was in the tomb for 3 days and 3 nights.

 Just how close to the 73^rd hour (or the beginning of the 4^th day) was he resurrected?

Was it around the 9^th hour of the 3^rd day? Was it around the 12^th hour of the day?

We do know that Jesus was resurrected on the 3^rd day. We know he had to be resurrected before the 4^th day or before the 73^rd hour had commenced.

Consequently, He was raised no later than the completion of the evening (12^th hour) of the 3^rd day, which also was called the weekly Sabbath day.
 

Clearly Christ would be raised on the 3^rd day.  Reference: Matthew 16:21, 17:23, 20:19; Mark 8:31, 9:31, 10:34; Luke 9:22, 18:33, 24:7; John 2:19 (21); I Corinthians 15:4.

Why did Christ have to be raised on the 3^rd day; why the requirement for 3 days and 3 nights?

Read Acts 2:31, “He seeing this before spake of the resurrection of Christ, that His soul was not left in hell (the grave), neither his flesh did see corruption.” Peter spoke, quoting from Psalm 16:10 “For thou will not leave my soul in hell: neither wilt thou suffer thine Holy One to see corruption.” 

The word corruption comes from the Greek diaphthora meaning decay or corruption.

Notice Acts 13:33-37. “God hath fulfilled the same unto us their children, in that he raised up Jesus again; as it is written in the second psalm, Thou art my Son, this day I have begotten thee. And as concerning that he raised him up from the dead, now no more to return to corruption, he said on this wise, I will give you the sure mercies of David. Wherefore he saith also in another psalm, Thou shalt not suffer thine Holy One to see corruption. For David after he had served his own generation by the will of God, fell on sleep and was laid unto his fathers and saw corruption: But he whom God raised up saw no corruption.”

Conclusion – Jesus (His flesh) saw no corruption, no decay. For three days and three nights Christ was in the heart of the earth, in (hell) the grave.

If Jesus were in the tomb longer than 3 days and 3 nights, into the 4^th day, he would have begun to decay. Consequently, if Jesus were in the tomb longer than three days and three nights, he (His flesh) would have seen corruption.

Notice John 11:17. “Then when Jesus came, he found that he had lain in the grave four days already.” Now verse 39, “Jesus said, Take ye away the stone. Martha the sister of him that was dead, saith unto him, Lord, by this time he stinketh: for he hath been dead four days.”

Lazarus was beginning to stink because he was beginning to see corruption/decay as he was now in the tomb (grave) four days.

If Christ were to have remained dead into the fourth day, beyond the third day, he would not have fulfilled David’s prophecy pertaining to the Holy One and corruption because Jesus would have begun to see corruption.

3 Days and 3 Nights were required. No more and no less.