When we’re keeping the biblical feasts it’s helpful to understand God’s definition of a Day: when we move from one day to the next. As we get this right, we’re getting a little closer to aligning with God’s rhythm and calendar.
There are a few obvious choices: midnight, as observed in the West; sunset or evening, as observed in Judaism; and sunrise or morning, as in some agricultural societies.
The traditions of Judaism are grounded in a biblical precedent for starting the day in the evening: God consistently identifies a day as the evening and the morning (Ge 1:5, 8, 13, 19, 23, 31), suggesting evening is followed by morning within this daily cycle.
And since the very first day began in with darkness first, God creating the heaven (space), Earth and water before creating light (Ge 1:1-2), which all occurred all on the first day (Ex 20:11), Day 1 must have ended with a transition from light to darkness to begin Day 2, which must also then have begun with a dark period, followed by a period of light. This pattern of starting a day in darkness would then necessarily have continued every day since: starting each day in the evening around sunset, as light fades into darkness.
And thousands of years later, as Moses taught Israel to keep sabbath, even enforcing the death penalty upon those who willfully violated it (Nu 15:32, 35), and since God tells us to work the remaining six days (Ex 20:9), He certainly instructed His people as to when the sabbath day begins and ends, if there was ever any question about it, so they would all be sure to cease from work for the entire sabbath day and not put the nation in danger by breaking their covenant with God.
And still thousands of years later, as God walked the earth in the Person of His beloved Son, though He violated much Jewish tradition, man-made laws wrongly imposed on His people, He never did violate His own Sabbath command (1Pe 2:22), nor did He challenge their understanding of a Day, when it started or ended, which they had evidently been practicing ever since Torah was given at Sinai.
Challenging the Jewish understanding of a Day is essentially claiming it has been incorrect during the entire history of Israel as a nation, from the time of Moses right down through the lives of Christ and the apostles, and that no prophet of God ever called them out on this or corrected it. It implies they were all constantly violating sabbath, ignorantly breaking God’s law by working on sabbath, violating their covenant relationship with God every single week. It’s inconceivable, as careful as God encourages us to be in observing His laws (Ps 119:4), that Christ Himself would have made this basic kind of error Himself, not pointing it out and neglecting to instruct His people in how to observe sabbath correctly.
We may be certain that Jesus Christ, as well as all of His apostles, approved of and lived by the Jewish definition of a Day, and so should we.
This is why we begin to honor Sabbath at sunset, and why we start celebrating biblical feasts with an evening meal.