The Beginning of the Scriptural Day
The typical Scriptural day consisted of one 24 hour period, beginning with 12 hours of darkness, and ending with 12 hours of light. This is seen in the record given of Creation, when each day’s work concludes with the statement: “And the evening and the morning were the first day.”
Gen. 1:5, etc.
“Evening” in scripture is defined in several places. “There thou shalt sacrifice the passover at even, at the going down of the sun, at the season that thou camest forth out of Egypt.” Deut. 16:6. “And at even, when the sun did set, they brought unto him all that were diseased, and them that were possessed with devils.” Mk. 1:32. Thus from when the sun did set, until the sun sets again, is a Scriptural day.
The “morning” is also defined. “And it shall be, that in the morning, as soon as the sun is up, thou shalt rise early, and set upon the city:” Judg. 9:33. “In the morning, when it is day, we shall kill him.” Judg. 16:2. “And he shall be as the light of the morning, when the sun riseth, even a morning without clouds;” 2 Sam. 23:4.
This is why we as Seventh-Day Adventists observe the Sabbath (Saturday) from sunset Friday evening, until sunset Saturday evening. "From even unto even, shall ye celebrate your sabbath." Lev. 23:32