Translations of the Torah into Aramaic, the vernacular of the time - the authoritative versions were ascribed to Onkelos (on Torah) and Yonatan ben Uziel (on Prophets). [Who's Who - for both of them] Also, the place of reading the megillah in comparison to other mitzvot, from learning Torah to caring for a "meit mitzvah" to welcoming a bride. Plus, the factor of publicizing a miracle.