Exodus 31:13

Also, speak thou with the sons of Israel, saying, Ye shall keep the day of My Sabbaths indeed; for it is a sign between My Word and you, that you may know that I am the Lord who sanctify you.

ACH’ (BUT) YE SHALL KEEP MY SABBATHS. “Even though you may be anxious to do the work promptly, do not set aside the Sabbath on its account. All [Scriptural expressions containing] the words ach (but) or rak (only), intimate limiting qualifications; [in this case] it is to exclude the Sabbath from [the days on which] the work of the Tabernacle may be done.” This is Rashi’s language. But I have not been able to explain it. For according to the method used by our Rabbis to interpret the words ach (but) and rak (only), it should limit the scope of Sabbath-observance [and permit the work of the Tabernacle on the Sabbath], for the limiting qualifications [of ach and rak] everywhere apply to the subject of the commandment itself [and since the verse here speaks of Sabbath-observance, the word ach should be restricting its application]. Thus if we are to interpret [and apply] the limiting nature of the word ach to the subject of the work of the Tabernacle, it should follow that it is allowed to be done on the Sabbath! Rather, the limiting nature of the word ach here applies to circumcision [on the eighth day of the child], or to the saving of human life, and similar instances, [to tell us] that they set aside the Sabbath. And so the Rabbis said in Tractate Yoma: “And whence do we know that even in a doubt whether life is in danger, the Sabbath may be set aside? Rabbi Abohu said in the name of Rabbi Yochanan: ‘Ach’ ye shall keep My Sabbaths — [the word ach] has a limiting qualification.” And the reason why the work of the Tabernacle does not override the Sabbath [is not on account of the word ach, but] because He warned [about keeping the Sabbath] here [right next to the subject of the making of the Tabernacle, thus indicating that the Sabbath is not to be set aside on account of it]. And in line with the plain meaning of Scripture the verse states as follows: “You shall do the work of the Tent of Meeting, but My Sabbaths you shall keep forever.” And in the Torath Kohanim Section 3, 7:7. [the Rabbis have said as follows]: “I might think that the building of the Sanctuary overrides the Sabbath? Scripture therefore says, Ye shall keep My Sabbaths, and reverence My Sanctuary: I am the Eternal.”
Now the reason for the word Shabtothai [in the plural] is because the Sabbaths of the year are many. By way of the Truth, [the mystic lore of the Cabala], He commanded here …

אך את שבתותי, even work of constructing the Tabernacle must not be performed on the Sabbath.

כי אות היא, for the reason that you are abstaining from such categories of activities is that you are like Me, i.e. you are My people.

אך את שבתותי תשמורו, even though it was I Who commanded you to build the Tabernacle and to construct all its appurtenances, and it is to serve as a residence for My presence, this commandment does not override the restrictions associated with proper observance of the Sabbath. The reason is that the Sabbath is

אות היא ביני וביניכם, a symbol serving as a reminder of our mutual relationship. If you were to ignore this symbol by building a residence for Me on this day, there would be no point in doing so as I would not then take up residence in that Tabernacle.

אך את שבתותי שמורו, “but you must observe My Sabbaths;” Moses is to tell the people that although they might have thought that having been commanded to construct a residence for G–d on earth, completing it as soon as possible would override any restrictions that are part of the Sabbath, the Sabbath is not to be desecrated on account of building the Tabernacle.