When all the congregation saw that Aaron was dead - Moses and Eleazar were the sole witnesses of his departure (Num 20:28). According to the established law, the new high priest could not have been present at the funeral of his father without contracting ceremonial defilement (Lev 21:11). But that law was dispensed with in the extraordinary circumstances. The people learned the event not only from the recital of the two witnesses, but from their visible signs of grief and change; and this event betokened the imperfection of the Levitical priesthood (Heb 7:12).
they mourned for Aaron thirty days - the usual period of public and solemn mourning. (See on Deu 34:8).