See the similar account of Aaron’s death in Num 20.20. Thirty days seems to have been the normal length of time of mourning for a leader.

Then the days of weeping and mourning … were ended: this is quite redundant information. To make the text less repetitive, it may be placed at the beginning of the next paragraph (as in New Jewish Publication Society’s Tanakh). Contemporary English Version has a good model for this verse:

• The people of Israel stayed in the lowlands of Moab, where they mourned and grieved thirty days for Moses, as was their custom.

Quoted with permission from Bratcher, Robert G. and Hatton, Howard A. A Handbook on Deuteronomy. (UBS Helps for Translators). New York: UBS, 2000. For this and other handbooks for translators see here .