Many have written to The Refiner's Fire to inquire about the start of a "day" as outlined in the Bible. The bottom line is, there is no historical evidence that God's people have observed Shabbat in any way other than from "sundown to sundown." Some verses that prove this include a couple from John, which outline the rush to get his body off the cross/stake and buried before sundown because the “Sabbath” was near.
John 19:31 And the Yehudeans, because it was the eve, said, "These bodies should not remain on their stakes because the Shabbat is dawning. " For it was a high day, the day of the Shabbat, that they entreated from Peelatos that they might break the legs of those who were nailed to the stake and take them down.
John 19:41 Now there was a garden in that place that Y'shua was executed in, And in that garden a new tomb that a man had not yet been laid in it. 42. And they placed Y'shua there because the Shabbat was beginning and because the tomb was near.
The "day" comes from scriptures like the following:
Genesis 1:5 - "Yahweh called the light Day, and the darkness he called Night. So there was evening, and there was morning, one day."
Deuteronomy 16: "Neither should any of the meat which you slaughter in the evening on the first day stay all night until morning.
Nehemiah 13: 19 It came about that just as it grew dark at the gates of Jerusalem before the sabbath, I commanded that the doors should be shut and that they should not open them until after the sabbath. Then I stationed some of my servants at the gates so that no load would enter on the sabbath day.
2 Chronicles 2: 4 Here, I am about to build a house for the name of ADONAI my God, to dedicate it to him, and to burn before Him incense made of sweet spices; the house will also be for the continuing showbread and for the burnt offerings presented every morning and evening, on the shabbats, at every Rosh-Hodesh, and at the designated times of ADONAI our God. This is a perpetual regulation for Isra'el.
So a whole "day" is measured from dark - that is, sunset - to the next sunset. Therefore a full "day" is sunset, through all night, through all day, until the sun sets again. Therefore, the Sabbath begins when the sun sets on Friday, and it ends when the sun sets on Saturday. Of course, according to the Gregorian calendar, Saturday remains "Saturday" until midnight, because we must use modern civil time to determine our days which are measured from midnight to midnight....
There is another meaning of the word "day" as you can see in the scripture above: When it is light - that is, the sun is up - it is also called "day". So you must take care to determine what "day" is meant as you read the Bible. In Matthew 12:40, Yeshua is referring to "day" as in "daylight", when the sun is up. Therefore when He says "For just as Yonah was three days and three nights in the belly of the sea-monster, so will the Son of Man be three days and three nights in the depths of the earth", He means "3 daytimes, and 3 night-times". In other words, 3 full days.
One problem here is that the English word "day" means both a full 24 hour day, and it means the period in which the sun is up. Another problem is that any 24 hour period is considered a "day". To explain this, try to imagine that at sunset, you are starting your clock at zero. Then, the next evening, your clock will show 23 hours and about 56 minutes have passed when the sun sets again. Just at sunset, your clock reads zero again.
This is not any different than our civil world where the clock changes from 23:59 to 00:00 during the middle of the night. Society has simply chosen to determine the "day" as measured from midnight to midnight, whereas Yahweh said it is from sunset to sunset.
How we know that God's "day" begins and ends at sunset? There are several scriptures that reveal this fact, including the following:
Genesis 1: 5 So there was evening and there was morning, one day.
Notice "evening" was mentioned first, then morning....This was repeated throughout Genesis 1 and reiterated throughout the Torah.
Leviticus 23: 27 "The tenth day of this seventh month is Yom-Kippur; you are to have a holy convocation, you are to deny yourselves, and you are to bring an offering made by fire to ADONAI. 28 You are not to do any kind of work on that day, because it is Yom-Kippur, to make atonement for you before ADONAI your God. 29 Anyone who does not deny himself on that day is to be cut off from his people; 30 and anyone who does any kind of work on that day, I will destroy from among his people. 31 You are not to do any kind of work; it is a permanent regulation through all your generations, no matter where you live. 32 It will be for you a Shabbat of complete rest, and you are to deny yourselves; you are to rest on your Shabbat from evening the ninth day of the month until the following evening.
Nehemiah 13: 19 So when the gates of Yerushalayim began to grow dark before Shabbat, I ordered that the doors be shut; and I ordered that they not be reopened until after Shabbat. I put some of my servants in charge of the gates, to see to it that no loads be brought in on Shabbat. 20 The merchants and sellers of all kinds of goods spent the night outside Yerushalayim once or twice, 21 until I warned them, "Why are you spending the night by the wall? Do it again, and I'll use force against you!" From then on they stopped coming on Shabbat. 22 Then I ordered the L'vi'im to purify themselves and come and guard the gates, in order to keep the day of Shabbat holy. My God, remember this too for me, and have mercy on me in keeping with the greatness of your grace!