"And when the sixth hour was come, there was darkness over the whole land until the ninth hour." Mk 15:33
Some say that this gives the "missing" day. Creation was disordered at the crucifixion, and the timing of the day was confused. This is why the rites for Bright Saturday are all out of the normal order, to commemorate this.
So At the 6th hour on Friday, darkness fell, and sunset was the start of the new day (and still is in the Church). At the ninth hour, the son shone again, and presumably set as normal at the 12th hour, the start of the second day. Then Saturday evening, the sun set again, starting the third day, on which Christ rose.