en_tn/num/01/18.md

the first day of the second month

This is the second month of the Hebrew calendar. The first day is near the middle of April on Western calendars. See how you translated this in Numbers 1:1. (See: rc://en/ta/man/translate/translate-hebrewmonths and rc://en/ta/man/translate/translate-ordinal)

Then each man ... identified his ancestry. He had to name the clans and families descended from his ancestors

The second sentence means basically the same thing as the first and is added for clarification. (See: rc://en/ta/man/translate/figs-parallelism)

He had to name

Here "name" means to "say." AT: "Each man had to say" (See: rc://en/ta/man/translate/figs-idiom)