The Bible does not use calendar days of the week such as Saturday and Sunday. However, traditionally the Jewish Sabbath is said to run from Friday evening to Saturday evening forming the seventh day (day of rest). The Passover Sabbath occurs during the first month (called Nisan) of the Jewish Torah year which is not the same as the Jewish calendar year. Nisan corresponds to around Mar/April of the American Julian calendar rather than January.
Jesus was crucified during the Passover Sabbath.
Mark 15:25,33-34 says Jesus was crucified at the 3rd hour (9am) and died at the 9th hour (3pm). This took place on the preparation day which is the day before the Passover Sabbath.
As the feast of unleavened bread approached (Luke 22:1), which is also called the Passover, Judas planned his betrayal. When the day of unleavened bread came when the Passover lamb is to be killed, Jesus sent Peter to kill a lamb to prepare the Passover (Luke 22:7-8). They then ate it in the upper room as Christ last supper (Luke 22:1-22). Soon after his last supper Jesus was arrested (Luke 22:39, 47, 54). It is done at the beginning of the day of preparation or right before it. It is what we know as Thursday evening; thus, the probable beginning of Jesus three day reference. This supper is the basis for the Lord’s Supper or Communion.
The next morning at daylight they took Jesus to the Sanhedrin Council for trial (Luke 22:66). They then took him before Pilate (Luke 23:1). It is now early morning on the preparation day which will lead to his crucifixion at 9:00 AM that morning on what we know as Friday.
Jesus was placed in tomb on the day of preparation for Passover Sabbath (G4521, Sabbaton) per Matthew 27:62; Luke 23:54; and John 19:14, 31, 42 which is the day before the Sabbath (G4315, proSabbaton) per Mark 15:42. On the Sabbath (G4521, Sabbaton) the people rested (Luke 23:56) on what we know as Friday evening to Saturday evening and most likely until daylight on what we know as Sunday morning. For early in the morning on the first day of the week (G4521, also Sabbaton, here indicating interval between two Sabbath Days) the women came to the tomb and found Jesus gone (Matthew 28:1, 6).
Key Scriptural References: Mark 15:25,33-34; Matthew 27:59-64; John 13:1-5; John 19:14,31,42; Mark 15:42-43; Luke 22:1,7-8,15,20; Luke 23:52-56; Matthew 28:1, 6; Acts 20:7; 1 Corinthians 16:2