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):

• Just as God had spoken to the nation of Israel at Har Sinai, so too does He continue to speak to them (via Moses) from the Ko'desh haKodashim (Holy of Holies), through the Kruvim (Cherubs) atop the Aron (Ark):
  

  I will commune with you there, speaking to you from above the ark-cover, from between the two cherubs that are on the ark of Testimony. [In this manner] I will give you instructions for the Israelites. (Exodus 25:22)

  
  
• The commandments which Moses receives (24:12) on Har Sinai serve as a testimony to receiving the Torah and thus are to be kept in the Aron, the focal point of the Mishkan:
  

  ...put in the ark the Testimony that I will give you. (25:21)

  
  
• The anan (cloud of smoke) created by the Mizbayach ha'ktoret (altar) symbolizes the anan that covered Har Sinai (19:9, 24:15-18).

• The aish (fire) on the altar symbolizes the aish that descended on Har Sinai (24:17).