The word translated, "between the two evenings, used in Ex 12:6 has two schools of thoughts about its interpretation. One being from noon to sunset (or at what time one believe the sun is descending). The other defines it as that small gap of time from sunset to dark. My option is with the latter of the two.
Scripture says the passover was originally killed and eaten on/during the 14th and the Feast of Unleavened Bread was kept on the 15th, twenty four hours later:
Ex 12:6,8: it is killed the evening of the 14th and eaten "in that night", not the next day
Lev. 23:5,6: the passover is on the even of the 14th and the Feast of Unleavened Bread is the 15th, the next day, 24 hours later
Num. 9:5: they kept the full passover on the 14th. Reading on you see that the second passover, one month later on the 14th they are instructed to "KEEP it and EAT it", on the same day, the 14th.
Joshua 5:10,11: they kept the entire passover "on the 14th day of the month at eve." and ate unleavened cakes the next day.
Ezra 6:19,22: people returning to Jerusalem "kept the passover upon the 14th" and then kept the Feast of Unleavened Bread the next day
Looking back on the OT, where the origins of the Passover are presented, God gave the Israelites instruction on it. On the 14th of Nisan, at even, they were to kill a lamb. They took the lamb on the 10th (Ex12:3) and kept it until the 14th (v6) and kill it in the evening, placing it's blood on the posts of their houses (v7). The death angle came and took the firstborn, but PASSED OVER the house that had the lambs blood on them. The Israelites were to eat the lamb that night and nothing was to remain by daylight. They were still there, in Egypt, the next morning, still the 14th
Then, the morning of the 14th, the Isrealites burned what was left of the lamb, got stuff from the Egyptians and gathered up their stuff. That night, now the 15th, the day after the Passover, they left Egypt. Num. 33:3 shows it to be the 15th and Deut. 16:1 shows it was at night when they left. Now, if the Passover was on the 15th, how could they of left the night of the 15th if they were ordered not to come out of their homes until morning while the death angle passed over? Impossible. The Passover was the night before, at the beginning of the 14th.
Notice Num. 28:16, 17. "And IN the 15th day of the first month is the passover of the lord.", "And IN the 15th day of this month is the feast: seven days shall unleavened bread be eaten." In those two separate days are two separate occasions. One of the Passover and the other, the Feast of Unleavened Bread. If you keep reading you will see, on the 1st day of Unleavened Bread (the Feast, Sabbath, Holy Day, whatever you want to refer to it by), they were instructed to make sacrificial offerings, not the Passover Lamb, for this High Day, not the Passover. This was to be done in the morning, not the evening. This was offerings for the Feast of Unleavened Bread, on the 15th of Nisan.
Now, if you take the above information (Passover the 14th and Unleavened Bread beginning the 15th), if you don't agree, humor me, and put it into the time-line that all 4 gospels make, it fits, with all of them. Now you have three occasions: the Passover on the 14th, the Days of Unleavened Bread on the 15th and the weekly Sabbath.
Tues. Christ ate Passover with the disciples on the 14th (beginning part) when He instituted the New Covenant, was betrayed by Judas, arrested and taken to the high priest
Wed. Christ is crucified, died and His body was placed in a tomb before sunset. This was preparation day for the Feast of Unleavened Bread, not the weekly Sabbath.
Thur. First Day of Unleavened Bread (High Day, Holy Day, Sabbath, Feast, etc), Mt. 27:26 calls it the day after the "Day of Preparation".
Fri. After the "High Day", the women bought and prepared spices for the body before resting for the weekly Sabbath, beginning at sunset
Sat. The weekly Sabbath. They rested. Christ rose near sunset. This would be exactly a full three days and a full three nights, fulfilling the only sign He gave of His messiah-ship.
Sun. The women came to anoint Christ while it was still dark, but He was already gone.