“when Moses came down from Mount Sinai.” This was Moses’ seventh and last time down Mount Sinai. He had been up on the mountain with God for 40 days and nights (Exod. 34:28), the second time he had spent 40 days and nights on Mount Sinai (cp. Exod. 24:18). After this seventh time on Mount Sinai, Moses assembled all the material for the Tent of Meeting and had it set up, and it was set up on the first day of the first month in the second year of the wilderness journey, so Israel had been in the wilderness for just short of a year when it was set up (they had left Egypt on the fifteenth day of the first month of the year before).
[For more information on Moses’ seven trips up Mount Sinai, see commentary on Exodus 19:3.]