The time for doing this is, On the first day of the first month — This wanted but fourteen days of a year since they came out of Egypt. Probably the work was made ready just at the end of the year, so that the appointing this day gave no delay. In Hezekiah's time they began to sanctify the temple on the first day of the first month, 2 Chronicles 29:17. The new moon (which by their computation was the first day of every month) was observed by them with some solemnity; and therefore this first new moon of the year was thus made remarkable.
Related Commentaries of Exodus 40:2
Exodus 40:2
2 On the first day of the first month shalt thou set up the tabernacle of the tent of the congregation.