In the first aliyah, Moshe gathers (the word "Vayakhel") the Jewish people and starts telling them G-d's commandments. He begins with the commandment of rest on Shabbos and of not lighting a fire on it, followed by the commandment to donate toward the building of, and to the actual building, of the Mishkan (sanctuary), its vessels, and the holy garments of the Kohanim. (We learned about the requirements of the building of the Mishkan, and of the garments of the Kohanim, in the recent parshiyot.)
Rashi says that the commandment of Shabbos precedes the one about the Mishkan to show that resting on Shabbos supersedes the building of the Mishkan. The men and women come forward and donated to the building, generously of their free will.