John Gill Commentary Numbers 33:51

"Speak unto the children of Israel, and say unto them, When ye pass over the Jordan into the land of Canaan," — Numbers 33:51 (ASV)

Speak to the children of Israel, and say to them
What was to be said, being what concerned the whole body of the people:

when you are passed over Jordan into the land of Canaan ;
near to which they now were, and Moses was about to leave them; and therefore it was the more necessary to give them some instructions and directions what they should do, when they were come into it.