When did God give Moses the tabernacle instructions?
God gave Moses the tabernacle instructions while Israel was still camped in the wilderness of Sinai, as part of the giving of the Law. The instructions covered building the tabernacle, and the people completed that work before their forty years of wilderness wandering and entry into the Promised Land.
Les explains that in order to bring in the religious system of the Law, God added to the Ten Commandments all the rituals, regulations, rules, civil law, and stipulations for the priesthood. God then gives Moses the instructions for building the tabernacle, and they accomplish all that while they are yet down in the wilderness of Sinai. Once the tabernacle is finished, the Shekinah glory comes down upon it, and whenever the glory lifted and moved, they took down the tabernacle and followed it. That was the giving of the Law, and they carried it on after the 40 years of wilderness into the Promised Land. For years it sat as a temporary little tent north of Jerusalem until God gave David, and then through his son Solomon, permission to build a permanent dwelling place, Solomon's Temple. Les notes this was all part of the giving of the Law to the nation of Israel, not to the Gentiles, though as seen in Romans chapter 3, the Law covered the whole human race.