Le 23:3 Six days shall work be done: but the seventh day is the sabbath of rest, an holy convocation; ye shall do no work therein: it is the sabbath of the LORD in all your dwellings.
The weekly Sabbath occurs on the 7th day of each week, which is our Saturday. It is, we learn from this text, a time set apart by God for us to get together, rest, relax, and feast. Sabbath is definitely considered a feast day and a time of congregational assembly along with the other feasts.
Many Christians observe Sunday, the first day of the week, as the Sabbath. Some teachers trying to defend this practice argue that since Jesus rose from the dead on Sunday, and since the early Christians often met on Sunday, that God has replaced the Saturday Sabbath with a Sunday Sabbath, in the same way and for the same reasons that He has (in their view) replaced physical Israel with the Church. They therefore call Sunday, The Lord’s Day.
There is only one small problem with this line of thought: God never said this or even suggested it; there is no indication of this concept in the Bible. The only support for it is long-standing Christian tradition, which is evidently rooted in attempts by early Christian leaders to invent a new religion that claimed to follow the Jewish Messiah but which was also fully decoupled from Judaism. Their motivation does not appear to be in a concern for proper theology, but rather a means to escape the horrible persecutions inflicted upon the Jewish people — and upon anyone who acted like them, as the early disciples did as they followed Torah — after the destruction of the Jerusalem temple in 70 CE. No other rational explanation is available to explain the roots of this clearly unbiblical tradition.
The biblical definition of the day is found in Genesis, where God made heaven and earth on the first day (Ex 20:11), prior to creating light. This proves that the first 24 hour day began in complete and utter darkness, during which nighttime period the entire foundation of Creation was laid. (Ge 1:2) It follows then that the evening period, the twilight period between the daytime and the nighttime, marks the division between any two days.
Additional evidence for this design is found in God’s command to celebrate the feast of Atonement on the 10th day of the 7th month (Le 23:27), and He defines this day as a sabbbath period between two evenings. (Le 23:32) If the day doesn’t begin in the evening, then we cannot keep this feast on the 10th since it will require us to celebrate part of it on another day, either the 9th or the 11th.
This general view of the day is confirmed by the behavior of the disciples after the Crucifixion, who recognized the Sabbath began in the evening. (Lk 23:54) They took Christ’s body down from the cross and placed it in a tomb (Lk 23:52-53) without making proper burial preparations (Lk 23:55), then went home and rested that evening and the rest of seventh day in order to obey the Sabbath command (Lk 23:56), returning to the tomb first thing Sunday morning with the burial spices they had prepared. (Lk 24:1)
Thinking that a day is only the daylight period of each 24-hour cycle is inconsistent with Ex 20:11. God did not create only during 6 daylight periods: earth and heaven were created in darkness, before the first morning, during the first day. Thus God refers to 24-hour periods as days.
Starting the day later than the evening, say at midnight or when it’s darkest, implies daytime is sandwiched in between two half-night times during every 24 hour day, which appears inconsistent with the Creation account, as well as the Day of Atonement.
Christ tells us that the Sabbath is made for Man (Mk 2:27), for everyone, not just for the Jews. Yet some argue that it isn’t for Gentiles since it God says it’s a sign between Himself and Israel (Ex 31:17), and also a memorial of their deliverance from slavery in Egypt. (De 5:15) but when He tells Israel to remember the sabbath, He bases it on the fact that He sanctified it on the seventh day of Creation.