For those who have never heard of “Lunar Shabbat”, there are certain Messianic/Hebrew Roots types who insist that the weekly Shabbat (Sabbath) does NOT fall on the Seventh Day of the week; rather, that it is determined by the New Moon each month.
They believe Shabbat always falls on the 8th, 15th, 22nd, and 29th of the month, and claim that ALL instances in the Bible where a Shabbat can be dated or the date inferred, it is always the 8th, 15th, 22nd, and 29th of the month - and cannot be proven otherwise!
Well, that is totally untrue – and my late husband, Bill Welker, proved it both biblically and scientifically. (Here is his whole article, if you are interested in doing a deeper study on this issue: Lunar Shabbat – NOT! )
So, let’s begin by checking out some of the basic “Sabbath” scriptures that seem to be ignored by the lunar sabbatarians:
In Genesis 1:1-31, and Genesis 2:1-3, we see that YHWH Himself established, blessed, and separated the Seventh Day as “holy”:
Genesis 2: Thus the heavens and the earth were finished, along with everything in them. 2 On the seventh day God was finished with his work which he had made, so he rested on the seventh day from all his work which he had made. 3 God blessed the seventh day and separated it as holy; because on that day God rested from all his work which he had created, so that it itself could produce. (CJB)
Exodus 20:8 "Remember the day, Shabbat, to set it apart for God. 9 You have six days to labor and do all your work, 10 but the seventh day is a Shabbat for ADONAI your God. (CJB)
Exodus 31:16 The people of Isra'el are to keep the Shabbat, to observe Shabbat through all their generations as a perpetual covenant. 17 It is a sign between me and the people of Isra'el forever; for in six days ADONAI made heaven and earth, but on the seventh day he stopped working and rested.'" (CJB)
Leviticus 23: 3 " 'There are six days when you may work, but the seventh day is a Sabbath of rest, a day of sacred assembly. You are not to do any work; wherever you live, it is a Sabbath to the LORD. (CJB)
Do you see any mention of the "new moons" in any of the scriptures above?
Here is the bottom line: ADONAI told His people to count SIX DAYS, and the Seventh would be the Sabbath.
Exodus 16:16 Here is what ADONAI has ordered: each man is to gather according to his appetite - each is to take an 'omer [two quarts] per person for everyone in his tent." 17 The people of Isra'el did this. Some gathered more, some less; 18 but when they put it in an 'omer-measure, whoever had gathered much had no excess; and whoever had gathered little had no shortage; nevertheless each person had gathered according to his appetite.
19 Moshe told them, "No one is to leave any of it till morning." 20 But they didn't pay attention to Moshe, and some kept the leftovers until morning. It bred worms and rotted, which made Moshe angry at them. 21 So they gathered it morning after morning, each person according to his appetite; but as the sun grew hot, it melted.
22 On the sixth day they gathered twice as much bread, two 'omers per person; and all the community leaders came and reported to Moshe. 23 He told them, "This is what ADONAI has said: 'Tomorrow is a holy Shabbat for ADONAI. Bake what you want to bake; boil what you want to boil; and whatever is left over, set aside and keep for the morning.'" 24 They set it aside till morning, as Moshe had ordered; and it didn't rot or have worms.
25 Moshe said, "Today, eat that; because today is a Shabbat for ADONAI - today you won't find it in the field. 26 Gather it six days, but the seventh day is the Shabbat - on that day there won't be any." 27 However, on the seventh day, some of the people went out to gather and found none. 28 ADONAI said to Moshe, "How long will you refuse to observe my mitzvot and teachings? 29 Look, ADONAI has given you the Shabbat. This is why he is providing bread for two days on the sixth day. Each of you, stay where you are; no one is to leave his place on the seventh day." (CJB)
The above passage makes it abundantly clear that something happened to the manna every day except on the sixth day: It would stink and breed worms if it was kept longer than a day! However, on the sixth day, whenever a double portion was gathered to last them through Shabbat (the Seventh Day) the manna did not spoil. On Shabbat Israel was instructed to remain in their tents and not attempt to gather manna because there would be none. Those who tried to gather on the seventh day were admonished.
Lastly, author and Aramaic and Hebrew scholar, Andrew Gabriel Roth, uses John 7 to completely squash the “Lunar Shabbat” argument:
John 7: 19. Did not Moshe give you Torah, yet not a man among you kept Torah? 20. Why do you want to kill me? The crowd answered and said, “You have a demon! Who wants to kill you?” 21. Y’shua answered and said to them, One work I have done and all of you marvel! 22. Because of this, Moshe gave circumcision to you. It was not because it was from him, Moshe, but from the patriarchs; yet you circumcise a man on the day of the Shabbat. 23. If a man is circumcised on the day of the Shabbat so that the Torah of Moshe not be loosed, why do you argue against me because the man I have made whole on the day of the Shabbat? 24. Do not be judging by hypocrisy, rather judge with a just judgment. (AENT)
 The Pharisees face a very stern rebuke here from Y’shua for not keeping the Torah of Moshe; his language could not be stronger. Y’shua’s teaching comes from the “divine” side (qnoma) within him. It is YHWH who is speaking through Mashiyach Y’shua. By contrast, “not a man among you,” not one Pharisee, is keeping Torah. The message is clear; Y’shua in effect is saying, “who are you to judge me when you don’t practice what you preach?” Even as a child from Galeela, Y’shua had a flawless understanding of Torah. Y’shua never lived one standard for himself while teaching another to the Pharisees, and another to his Jewish followers, and yet another for non-Jews. See Matthew 5:20.
 This event is in active participle form in Aramaic, indicating that it is unfolding into the immediate present. As a result, this verse clearly invalidates the false theory of a lunar Shabbat. This cannot be referring to a miracle on any other time but, rather, at that moment on that day – since that selfsame moment is called the Shabbat day!
Y’shua states that a day between the first and last days of Sukkot is the Shabbat. That being the case, the Shabbat is by definition, falling on a day other than the 1st, 8th, 15th, 22nd or 29th day of the month! This imperative clearly nullifies the lunar Shabbat falsehood; it is validated by Aramaicists and authoritative Aramaic grammars of both Christian and Jewish persuasion. Neither is there any historical documentation among Netzarim followers of Y’shua regarding the observance of a lunar Shabbat.
Nor do Jewish historians such as Yoseph ben Mattathias (Josephus) support the theory that Y’shua’s talmidim observed a lunar Shabbat. Had Y’shua or his talmidim adopted lunar Shabbat observance, the Rabbinical world would have raked them over the coals, but no such thing is evidenced in Jewish writings.
And so there you have all the proof you need about the SEVENTH DAY Sabbath!
(By the way, the "seventh day" on the Gregorian calendar falls on Saturday and it's the same 7th day observed since at least the time of Moshe! Please check out our article: Origin of the Seventh Day)