In the Sinai Desert, in the Tent of Meeting, etc.: That is why the verse needed to say, "in the Tent of Meeting in the second month." It is like a man who enters with his bride under the wedding canopy and writes her a ketubah, signing the time and the place to it. Likewise did the Holy One, blessed be He, do. As up until now, he wrote them neither a ketubah nor a divorce bill. But now he wrote them a ketubah, however he did not write them a divorce bill. That is what the prophet said (Isaiah 50:1), "Where is your mother's book of divorce?"
Sinai as a Wedding and the Marriage Contract With Israel
Midrash Aggadah, Numbers 1:1