- 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.
כא לְקַיֵּם עֲלֵיהֶם לִהְיוֹת עֹשִׂים אֵת יוֹם אַרְבָּעָה עָשָׂר לְחֹדֶשׁ אֲדָר וְאֵת יוֹם–חֲמִשָּׁה עָשָׂר בּוֹ בְּכָל–שָׁנָה וְשָׁנָה
21. To establish on them to make the doing of the fourteenth day of the month of Adar and the fifteenth day of it in every year and year.
- The Ginzei HaMelech notes that the word, kayam (“established”) is used ten times in Megillas Esther. This parallels the Ten Commandments in order to demonstrate that Purim is an extension of the fulfillment of the Torah, not an addition to it.
- In Chiddushei HaLeiv, R’ Hanoch Leibowitz quotes the Talmud Yerushalmi (Megillah 1:5) which says that 85 elders, among whom were 30 or more prophets, struggled with fitting the addition of Purim with the idea behind the words (Vayikra 27:34) “these are the mitzvos.” which do not allow for adding new mitzvos. R’ Leibowitz notes that these wise people were said to “struggle” over these words because that is what great leaders and Halachic deciders to – they struggle. He quotes the Chut HaMeshalesh (8) of R’ Chaim Volozhinner in which the author writes that he struggled to free a particular agunah (“married woman whose missing husband had not granted her a divorce”) to remarry. R’ Leibowitz notes that, beyond the intellectual logic and knowledge to figure out a law, a posek needs to have an overwhelming desire to help others, and to struggle for them, and this is why the verse stresses that such a leader – Mordechai – established these holidays for his flock.