Moses transferred priestly authority to Eleazar by giving him Aaron's priestly garments (compare Leviticus 8:7–9). After this, Aaron died at 123 years old, in the fortieth year after the Hebrews left Egypt (see Numbers 33:37–39).
the entire house of Israel mourned for him thirty days: This was a great honor (see also Deuteronomy 34:8). The usual public mourning period was seven days (Genesis 50:10; 1 Samuel 31:13).