Numbers 1:18

And they assembled all the congregation together on the first [day] of the second month,.... The month Ijar, as the Targum of Jonathan, answering to part of April and May: this was done on the selfsame day the Lord spake unto Moses about this affair, Numbers 1:1; so expeditious were he and Aaron in doing the will of God:

and they declared their pedigrees; either Moses and Aaron, according to Aben Ezra, who inquired when they were born, because of the computation of twenty years; and then their birth was wrote down, as he says; or rather the people declared their pedigrees, of what tribe, family, and house they were, who their parents, when born, and so, of course, how old they were; Jarchi interprets it, they brought the books of their genealogies, and witnesses to confirm the birth of everyone of them, to show their genealogy according to their tribe; nor is it at all unlikely that every family and house, or master thereof, kept a register of those born to him in it, whereby their age could be ascertained as well as pedigree:

after their famines, by the house of their fathers, according to the number of their names, from twenty years old and upwards, by their poll: that is, every tribe gave an account of the families in it, every family what houses were in it, and every house what number of males were in it, and of what age; and such were numbered who were twenty years old and upward.