Charles Ellicott Commentary Deuteronomy 1:3-4

"And it came to pass in the fortieth year, in the eleventh month, on the first day of the month, that Moses spake unto the children of Israel, according unto all that Jehovah had given him in commandment unto them; after he had smitten Sihon the king of the Amorites, who dwelt in Heshbon, and Og the king of Bashan, who dwelt in Ashtaroth, at Edrei." — Deuteronomy 1:3-4 (ASV)

Moses spoke to the children of Israel ... after he had slain Sihon ... and Og. — The conquest of these two kings and their territories was one of the exploits of the fortieth year . Before the eleventh month of that year, not only Sihon and Og, but also the five princes of Midian, who were dukes of Sihon, dwelling in the country (Joshua 13:21), had also been slain (Numbers 31:0). This completed the conquest and was the last exploit of Moses’ life.

In the period of repose that followed, he found a suitable time to exhort the children of Israel, according to all that the Lord had given him in commandment to them. From Deuteronomy 34:8, we learn that the children of Israel wept for Moses thirty days. These days would seem to be the last month of the fortieth year, for on the tenth day of the first month (Joshua 4:19), they passed over Jordan. Thus, the last delivery of the discourses recorded in Deuteronomy would seem to lie within a single month.