John Gill Commentary Numbers 33:38

"And Aaron the priest went up into mount Hor at the commandment of Jehovah, and died there, in the fortieth year after the children of Israel were come out of the land of Egypt, in the fifth month, on the first day of the month." — Numbers 33:38 (ASV)

And Aaron the priest went up into Mount Hor, at the commandment of the Lord Delivered to Moses:

and died there in the fortieth year after the children of Israel were come out of Egypt ; not being suffered to go with them into the land of Canaan, because of his sin of unbelief at Kadesh, the last place from which they came: in Mount Hor he died,

on the first day of the fifth month ; the month Ab, answering to part of July and part of August; so that he lived but four months after his sister Miriam; see (Numbers 20:1Numbers 20:25–29) .