The Ramban explains that the commandment to build the Mishkan was actually the first and primary mitzvah that God gave Moses during the forty days on Har Sinai. The Mishkan, Ramban explains (see 25:1), serves as a vehicle to perpetuate the Sinai experience. Therefore, it is the first mitzvah which God gives Moses when he ascends Har Sinai.

The Ramban's argument explains the numerous similarities between the Mishkan and Ma'amad Har Sinai (when Moses receives the ten commandments at Mount Sinai):