Kommentar zu Dewarim 1:3

ויהי בארבעים שנה בעשתי עשר חדש באחד לחדש AND IT CAME TO PASS IN THE FORTIETH YEAR, IN THE ELEVENTH MONTH, ON THE FIRST OF THE MONTH, [MOSES SPAKE] — This tells us that he reproved them only shortly before his death (Jewish tradition holds that Moses died on the seventh day of the twelfth month; cf. Megillah 13b). From whom did he learn this? From Jacob, who reproved his sons only shortly before his death. He said, “Reuben, my son, I will tell you why I have not reproved you for your unfitial conduct during all these years: it was in order that you should not leave me and go and join Esau, my wicked brother". — And on account of four things one should not reprove a person except shortly before one's death: that one should not reprove him and again have to reprove him; and that his fellow whom he reproves should not, when he afterwards happens to see him, feel ashamed before him, etc.; as it is set forth in Siphre. And similarly, Joshua reproved Israel only shortly before his death (cf. Joshua 24:1—29), and so, too, Samuel, as it is said, (I Samuel 12:3) “Behold, testify against me", and so, also, David reproved his son Solomon only shortly before his death (cf. 1 Kings 2:1—9).

ויהי בארבעים שנה, after the men slated to die in the desert had finished dying.

ויהי בארבעים שנה בעשתי עשר חודש, “It was in the fortieth year, during the eleventh month, etc.” Moses rebuked the people when he was already close to death; you find that our patriarch Yaakov also waited until he was close to death before rebuking those of his children whom he wanted to rebuke (Genesis chapter 49). Joshua also waited until he was close to death before he rebuked the people (Joshua chapter 24). The prophet Samuel was also close to death when he challenged the people to tell him if they had any complaints against him (Samuel I 12,2-3). King David also waited with telling his son Solomon who needed to be rebuked (Kings I 2,1).