A more detailed account of the death of Aaron is found in 20.22-29.

And Aaron the priest went up Mount Hor at the command of the LORD …: See 20.27. It may be important in some languages to mark verses 38-39 as a comment that has been inserted into the journey list (travel narrative) by means of a shift in word order (so Good News Translation) or through the use of an appropriate transitional expression, such as “It so happened that…” (Chewa). For Aaron the priest, see the comments on 3.6. At the command of the LORD is literally “upon the mouth of the LORD” (see verse 2 and 3.16).

And died there, in the fortieth year after the people of Israel had come out of the land of Egypt, on the first day of the fifth month: Aaron died along with the entire rebellious generation of Israel. He died in the fortieth year, the last year of their wilderness wandering (see 14.33; 32.13). On the first day of the fifth month is only the second explicit date mentioned in the journey list (after that at the beginning in verse 3), which serves to suggest the importance of Aaron’s death and its timing. The fifth month in the Hebrew calendar corresponds to mid-July to mid-August.

And Aaron was a hundred and twenty-three years old when he died on Mount Hor: According to Exo 7.7, Aaron was 83 years old just before the exodus from Egypt.

New Living Translation provides the following helpful model for verses 38-39:

• 38 While they were at the foot of Mount Hor, Aaron the priest was directed by the LORD to go up the mountain, and there he died. This happened in midsummer, on the first day of the fifth month of the fortieth year after Israel’s departure from Egypt. 39 Aaron was 123 years old when he died there on Mount Hor.

Quoted with permission from de Regt, Lénart J. and Wendland, Ernst R. A Handbook on Numbers. (UBS Helps for Translators). Miami: UBS, 2016. For this and other handbooks for translators see here .