In the Bible the days always begin and end at sunset.
This can be seen in the creation story in Genesis 1:
3. And God said, Let there be light: and there was light.
4. And God saw the light, that it was good: and God divided the light from the darkness.
5. And God called the light Day, and the darkness he called Night. And the evening and the morning were the
first day. Genesis 1.3-5
And in the fasting regulations for The Day of Atonement:
27. Also on the tenth day of this seventh month there shall be a day of atonement: it shall be an holy
convocation unto you; and ye shall afflict your souls, and offer an offering made by fire unto the Lord.
28. And ye shall do no work in that same day: for it is a day of atonement, to make an atonement for you before the
Lord your God.
29. For whatsoever soul it be that shall not be afflicted in that same day, he shall be cut off from among his people.
30. And whatsoever soul it be that doeth any work in that same day, the same soul will I destroy from among his
31. Ye shall do no manner of work: it shall be a statute for ever throughout your generations in all your dwellings.
32. It shall be unto you a sabbath of rest, and ye shall afflict your souls: in the ninth day of the month at
even, from even unto even, shall ye celebrate your sabbath. Leviticus 23.27-32
The 10th day of the seventh month is The Day of Atonement, in Hebrew the "Yom Kippur".
It starts at sundown of the 9th, and continues until sundown of the 10th.
This is why the religious leaders of the Jews wanted Christ's body off the cross before sunset, the
Passover was going to start at sunset:
31. The Jews therefore, because it was the preparation, that the bodies should not remain upon the cross
on the sabbath day, (for that sabbath day was an high day,) besought Pilate that their legs might be broken,
and that they might be taken away. John 19.31
At sundown the new day would start.