In פרשת כי תשא, we have the source for the 39 מלאכות of Shabbat. These 39 are viewed so seriously that the intentional violation of any one of them with warning and witnesses, gives the violator the death penalty. If he knew it was forbidden but there were no witnesses, he would get כרת. And if he violated any of these 39 by mistake, he would need to bring a sin offering, קרבן חטאת.
The previous Parshiot of תרומה and תצוה and the first part of כי תשא, all discussed details involved in the construction of the Mishkan. Suddenly, we are presented with the Pasuk, אך את שבתותי תשמורו, "But, you shall observe My Shabbatot. The word אך has significant derivatives where it is a word that teaches a great deal. (More examples of אך tomorrow, G-d willing) In this case the אך teaches that the construction of the Mishkan cannot take place on Shabbat and even more importantly, those 39 acts done in order to construct the Mishkan, are the same 39 acts forbidden to be done on Shabbat.