These … are the generations of Aaron and Moses. The word "generations" (toledoth) is used here in a peculiar and, so to speak, technical sense, with reference to what follows, as in Genesis 2:4; Genesis 6:9.
It marks a new departure, looking down, not up, the course of history. Moses and Aaron were a beginning in themselves as the chosen heads of the chosen tribe: Moses having the higher office, but one entirely personal to himself; Aaron being the first of a long and eminent line of priests.
The actual genealogy, therefore, is that of Aaron, and he is placed first. In the day. Apparently the day mentioned in Numbers 1:1; or it may be more general, as in Genesis 2:4.