וַיְהִי֩ בַיּ֨וֹם הַשְּׁלִישִׁ֜י בִּֽהְיוֹתָ֣ם כֹּֽאֲבִ֗ים וַיִּקְח֣וּ שְׁנֵֽי־בְנֵי־יַ֠עֲקֹב שִׁמְע֨וֹן וְלֵוִ֜י אֲחֵ֤י דִינָה֙ אִ֣ישׁ חַרְבּ֔וֹ וַיָּבֹ֥אוּ עַל־הָעִ֖יר בֶּ֑טַח וַיַּֽהַרְג֖וּ כָּל־זָכָֽר׃
And it was on the third day, when they were in pain, that two of the sons of Yaakov, Shimon and Levi, brothers of Dinah, each took his sword and came upon the city securely, and they killed every male.
Torah Temimah points to Bereshit Rabbah 80:10.
Sefaria · Bereshit Rabbah 80:10
ביום השלישי — כשהכאב גדול ביותר.
“On the third day” — when the pain is greatest.
The Midrash notes the timing of the attack when the pain of circumcision was at its height.
Derash on the timing (Bereshit Rabbah).
Shimon and Levi chose the third day, when the pain of circumcision is most intense.