So When Does The Sabbath Really Begins and How Long Is It?

A day is 12 hours … Sabbath DAY, NOT Sabbath NIGHT …
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Genesis 1
5 And God called the light DAY, and the darkness he called Night. And the EVENING and the MORNING were the FIRST DAY.
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Genesis 1
16 And God made two great lights; the greater LIGHT to RULE the DAY, and the lesser LIGHT to rule the NIGHT : he made the stars also.
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Exodus 20
10 But the seventh DAY is the sabbath of the Lord thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates:
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Exodus 16
22 And it came to pass, that on the sixth day they gathered twice as much bread, two omers for one man: and all the rulers of the congregation came and told Moses.
23 And he said unto them, This is that which the Lord hath said, Tomorrow is the rest of the holy sabbath unto the Lord: bake that which ye will bake to day, and seethe that ye will seethe; and that which remaineth over lay up for you to be kept until the morning.
24 And they laid it up till the morning, as Moses bade: and it did not stink, neither was there any worm therein.
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Matthew 28
1 In the end of the sabbath, as it began to DAWN toward the first day of the week, came Mary Magdalene and the other Mary to see the sepulchre.
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Luke 24
1 Now upon the first day of the week, very early in the morning, they came unto the sepulchre, bringing the spices which they had prepared, and certain others with them.
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John 11
9 Christ answered, Are there not twelve hours in the day? If any man walk in the day, he stumbleth not, because he seeth the light of this world.
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Mark 15
42 And now when the even was come, because it was the preparation, that is, the day before the sabbath,