As you are aware the Hebrew culture’s sense of timing was and as far as I know still different from ours. This is certain during the time of the disciples who would have kept to Hebrew timing.
At the time of the crucifixion Jewish law had an important legal stipulation. This stated that a person was not legally dead until a full three 24-hour day’s had passed i.e. 72 hours. Because of our system of timing, inherited from the R.C. church, we assume that “early the first day of the week” meant Sunday morning sunRISE. According to the Companion Bible and its appendages this verse in Mark’s gospel meant Saturday evening just as the sun set. There is a most interesting discussion in these appendages on the right day of the Passover at the time of the crucifixion, claiming it was on a Wednesday not a Friday.
In order to have the Lord crucified on Passover and prevent the women from preparing His body for burial, He had to be crucified on Thursday. It was called preparation day in the Lord’s time because at sunset the Feast of unleavened bread began, so all the preparation had to be done beforehand. That made Friday the special Sabbath that John spoke of (John 19:31). Then Saturday was the normal Sabbath. No work could be done on either day, keeping the women away. Had the Lord been crucified earlier in the week the women could have come the next day and no one would have noticed when He rose. Finally on Sunday Morning the women could come to anoint the body and discover that the Lord had risen. The Bible doesn’t say He rose at sunrise, but that the women got there at sunrise and found the empty tomb.
I don’t know where you got the information about the 72 hour rule, but I think someone has embellished the Hebrew tradition that a deceased person’s soul lingered near the body for 3 days. On the fourth day the person was considered dead and that’s when they believed the body began to decompose. It was also normal to consider any part of a day as a full day when counting. Therefore the three days and three nights prophecy can be fulfilled in less than 72 hours.
With night preceding day in the Hebrew manner of counting, the three days would have been Thursday, Friday and Saturday, and the three nights would have been Friday, Saturday, and Sunday. Thursday day and Sunday night were slightly shortened periods.
Psalm 16:10 contains a prophecy that God would not leave His Holy One in the grave nor allow his body to see decay. Both Peter (Acts 2:27) and Paul (Acts 13:35) applied this to the Lord. This confirms that Jesus had to come out of the grave before decomposition could begin on the fourth day, and lends support to a 3 day, 3 night stay of less than 72 hours.
This also explains why Jesus delayed before coming to raise Lazarus (John 11:6). Had he called Lazarus from the grave earlier, people could have claimed that Lazarus wasn’t really dead. By waiting till the 4th day, He removed all doubt. (John 11:39)