I do not read ANYTHING in the Gospel accounts or other places in scripture where such an accounting exists in relation to Jesus being in the grave.
Jesus specifically said "Three days AND Three nights" would be the ONLY SIGN given of His Messiahship.
"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" - Matthew 12:40.
Three days and three nights in the heart of the earth or grave. Jesus DID NOT include the suffering in the Garden 9 hours previous to his crucifixion in the accounting.
For folks who rightfully insist on going by every plain word in the Constitution to mean what it says - why is the same concept so hard to apply with the scriptures?
The confusion ALWAYS begins when you attempt to shove traditions and agendas into the plain reading the the bible.
The Left does this all the time with the Law of the Land, and reads their own agendas into the plain words of our Constitution. Just ask a public high Schooler to recite the First Amendment, they will tell you "Separation of church and state". Such is nowhere found in the plain words written, but agendas and traditions READ INTO the plain words, totally changes what the vast majority understand what those plain words meant by those who wrote them.
The bible has suffered this same fate millennia ago, until most have no clue what the plain words mean outside of tradition that they were taught as children. When confronted with the original intent and plain meaning of the words, pretzel logic always ensues and men look to other places to cobble together a justification for the discrepancy between agendas/traditions and the plain words.
This is how applying a private letter Jefferson wrote in 1801, when he was not even one of the writers or signatories to the Constitution, changes the meaning of First Amendment to mean something entirely different than written. So too - this is how traditions are yanked from other places to make the bible work within traditions grafted INTO the church.
Three days AND Three nights - it's not that difficult to understand. The secularists bash Christians over the head with traditions read into scripture to prove our faith a myth. Yet the bible interprets itself and is most often deliberately taken out of context to prove a point or push an agenda - same as our Constitution is now suffering. Until a time comes when few even recall what the original words really meant.
"My people are destroyed for a lack of wisdom" - Hosea 4:6
So that means, to correct you, that includes Friday to Sunday, three days.
I also believe that from death to resurrection it was literally "three days and three nights" (not "one day and two nights" as some wish to rationalize it). Jona's duration in the belly of the great fish was measured by Jesus as a "three days and three nights" duration.
However, I do not think "heart of the earth" is the grave. Peter's sermon in Acts 2:29-32 says:
29* Men and brethren, let me freely speak unto you of the patriarch David, that he is both dead and buried, and his sepulchre is with us unto this day. 30* Therefore being a prophet, and knowing that God had sworn with an oath to him, that of the fruit of his loins, according to the flesh, he would raise up Christ to sit on his throne; 31* He seeing this before spake of the resurrection of Christ, that his soul was not left in hell, neither his flesh did see corruption. 32* This Jesus hath God raised up, whereof we all are witnesses.
I think the heart of the earth is in fact hell. And Christ's soul was there three days and three nights, while his body (flesh) was in the tomb where it did not see corruption.
Again, your opinion, not mine.