What month was the first day of the first month when the tabernacle was set up?

The tabernacle was set up on the first day of the first month, which was April. This was in the second year after the Exodus, as God instructed Moses to erect the tabernacle and place the ark of the testimony within it, with all its furnishings prepared by the craftsmen.

Les explains that they are still in the first 12-month period since leaving Egypt, gathered around Sinai. The Lord spoke to Moses in Exodus 40:1, saying, "On the first day of the first month..." Les notes they came out of Egypt the night of the Passover in the first month, so this is the next first of the next year. He identifies that first day of the first month as April. God instructed Moses to set up the tabernacle of the tent of the congregation, put the ark of the testimony therein, cover it with the veil, and do all that He commanded. By verse 33, they had made everything functional, set up the tabernacle with all furnishings, and reared up the court, the linen wall.

Exodus 40:2

On the first day of the first month shalt thou set up the tabernacle of the tent of the congregation.

On what date was the tabernacle to be set up in Exodus 40?

Exodus 40:1-2 specifies the tabernacle was to be erected on the first day of the first month, which the lesson identifies as April. This occurred in the next year after Israel left Egypt, during the first twelve-month period at Sinai.