Immediately and miraculously, which was done not only to procure the greater reverence to the law, but also to signify that it was the work of God alone to write this law upon the tables of men's hearts. See Jeremiah 31:33 2 Corinthians 3:3,7. In the day of the assembly, i.e. when the people were gathered by God's command to the bottom of Mount Sinai, to hear and receive God's ten commandments from his own mouth.
Related Commentaries of Deuteronomy 9:10
Deuteronomy 9:10
10 And the LORD delivered unto me two tables of stone written with the finger of God; and on them was written according to all the words, which the LORD spake with you in the mount out of the midst of the fire in the day of the assembly.