קִיְּמ֣וּ וְקִבְּל֣[וּ] הַיְּהוּדִים֩

the Jews fulfilled and accepted

says the Megillah (Est 9:27), which is darshened “they fulfilled that which they accepted already.” (Shavuos 39a)

Purim not only introduces the redemption season, it represents the completion of the redemption process. Pesach’s freedom finds its purpose in Shavuos’s Torah. And the acceptance of mitzvos of Shavuos is finally fully embraced on the first Purim.

Pesiqta Zutera (end of Rus) says that we read Megillas Rus on Shavuos because “שמגילה זו כולה חסד, והתורה כולה חסד – this megillah [Rus] is entirely chessed, and the Torah is entirely chessed.”

The same is true of the four mitzvos of Purim. One is reading the megillah, commemorating the meaning of the day. And the other three? Matanos le’evyonim – gifts to the needy, and mitzvos about sharing food, the ties that bind people into community: mishloach manos – gifts of food, and the Purim feast. Purim’s mitzvos use the miracle of 2,475 years ago to motivate our concern and connection with other people. So that no enemy again can accuse us that “

Shavuos is about chessed. Purim is about chessed leading to achdus. As Rav Shimon Shkop put it (Intro. to Shaarei Yosher; see further Widen Your Tent pp. 79 onward):

וחיי עולם נטע בתוכנו
שיהיה אדיר חפצנו להיטיב עם זולתנו

“… and planted eternal life [the Torah] within us,
so that our greatest desire should be to benefit others.”

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The above is from the “tails” side of the “Happy Purim from…” note included in this year’s mishloach manos (“shalachmanos“). For a discussion of Purim in relation to Pesach and Shavous at more length, there is an earlier post “For the Jews, There Was Light” (2014).

Personally, I particularly like the question I discussed there: “Megillas Esther tells us ‘laYhudim haysa orah visimchah visason viykar — for the Jews there was light, happiness, joy and preciousness.’ Rabbi Yehudah (Megillah 13b) explains that orah (light) refers to Torah, simchah (happiness) is Yom Tov, sason (joy) is beris milah, and yeqar (preciousness) is tefillin. … So why didn’t the megillah simply say “for the Jews there were Torah, holidays, milah and tefillin. Why the code words?”