For Moses had said
To the Levites, when he first gave them their orders:
consecrate yourselves today to the Lord;
devote yourselves to his service, by obeying his orders, slaying those, or the heads of them, who have cast so much contempt upon him as to worship the golden calf in his room; and which would be as acceptable to him as the offerings were, by which Aaron and his sons were consecrated to the Lord; and as these Levites were consecrated to his service this day, on this account:
[even] every man upon his son, and upon his brother;
not sparing the nearest relation found in this idolatry, and for which the tribe of Levi is commended and blessed in the blessing of Moses, ( Deuteronomy 33:8 Deuteronomy 33:9 ) and as it follows,
that he may bestow a blessing upon you this day;
which was their being taken into the service of God to minister to the priests in the sanctuary, to bear the vessels of the Lord, and for their maintenance to have the tithes of the people: this day was, according to the Jewish writers F19, the seventeenth of Tammuz, or June, on which day the Jews keep a fast upon this account.
F19 Sedar Olam Rabba, c. 6. p. 18. Pirke Eliezer, c. 46.