Radak: Above (1:15) it says that they began on the 24th of the sixth month (Elul), i.e. to prepare materials for building. In the ninth month (Kislev) they began building, and that day Hashem told Chagai to ask the Kohanim laws of Tum'ah and Taharah. They offered Korbanos before this, from when they first ascended from Galus and built a Mizbe'ach, 19 years before the Bayis was built! Even so, Hashem did not tell him to ask before this, for the Korbanos were not fixed until the day that they started building ¹ .