Our parasha opens with Moshe telling Bnei Yisrael what Hashem had commanded in regard to the building of the Mishkan (Tabernacle). However, first it reiterates the commandment to refrain from forbidden work on Shabbat. There was a similar linkage between the Mishkan and Shabbat in Parashat Ki Tisa, at the end of Hashem’s commandment to Moshe about the Mishkan (Shemot 31:13), but here Shabbat is mentioned before everything.
Chazal (cited by Rashi, Shemot 35:2) learn from the linkage that despite the importance of the mitzva to erect the Mishkan, it did not justify desecrating Shabbat for that purpose. We can understand the philosophical significance of this halacha if we take into consideration the basic content of the mitzva of the Mishkan. According to several commentators, including Rashi, the mitzva originated only after the sin of the Golden Calf. The Seforno explains that originally Hashem just commanded, "An altar of earth erect for Me ... to every place that I shall mention My name, I will come to you and bless you" (Shemot 20:21); after the sin they would need kohanim to make the berachot.
Let us put this in broader perspective in the following way. There is a phenomenon of naming a specific place for service of Hashem and a specific tribe to be involved in it, but this was not what Hashem preferred. It would have been better with a simple altar, without gold and silver or special kohanim with their special clothes. Rather, every Jew would be a kohen, the whole Land would be a Mikdash, and Hashem’s blessing would come everywhere. After the sin, everything had to be more specifically chosen.
However, as much as the means through which one reached the goal changed, the goal itself did not, and that is: "and I [Hashem] will dwell in their [the people’s] midst" (Shemot 25:8). All the Mishkan did was to create a point around which they would focus, where they could act and learn how to incorporate Hashem into their lives. The public Mikdash is not to replace the private one. Heaven forbid, one should never think that what he does in the Mikdash protects him from a sinful life that he leads outside of it. That was a real danger that the prophets, including Yirmiyah (7:9-10), warned about.
For this reason, the Torah felt it necessary to stress with the building of the Mishkan the matter of keeping Shabbat. The people must know that Shabbat, the personal spiritual constant that applies to every Jew wherever he is, still fully applies. The building of the Mishkan will not change that. While it was enough for Hashem to mention Shabbat after the commandment of the Mishkan, Moshe was afraid that when telling Bnei Yisrael about the MIshkan, they might get so carried away by the excitement that they would forget what Hashem truly wanted. It is for this reason that the Torah started off with the warning to keep Shabbat.