The Vilna Gaon writes that the verse’s account of what the Jews “doing” refers to their feasting and joy, and it stresses that they did this on their own, without the need to be told to do so by Halachic rule.
- R’ Yehonason Eibshutz explains that what the Jews began doing was a continuation of what they started doing at Sinai. For this reason, although there is a prohibition (Devarim 13:1) to add mitzvos to the Torah, Mordechai had the authority to add the mitzvos of Purim in this case. The nation achieved atonement because they listened to Mordechai, and the sincerity of their re-acceptance of Torah allowed for the Torah to be new.
- The Sfas Emes and Maamar Mordechai both note that in this verse, too, the Jews did before they learned, in the same fashion as their saying (Shemos 24:7) “naaseh v’nishmah” (“we will do and we will listen”) when they first accepted the Torah.