Ester 9:17 Midrash: Midrash Tanchuma

What is the day of assembly? It was said: The thirteenth of the month of Adar is the day of assembly, as it is written: On the thirteenth day the Jews assembled in their cities (Est. 9:1–2). That is, they assembled on that day and decreed that the thirteenth day of Adar should be a fast-day, However, they designated the fourteenth day of Adar as a festive day, as it is written: And on the fourteenth day they rested and made it a day of feasting and gladness (ibid., v. 17).

The second occurred in the days of Mordecai and Esther. What is written there? The Jews ordained, and accepted it upon them and their seed (Est. 9:27). How do we know that the Holy One, blessed be He, concurred? Because it is written: the Jews ordained and accepted. The word accepted (kibal) is written in the singular form, thereby indicating that the Master of the Jews concurred.