The celebration of this event (2 Macc 15:36-15:36)

“They all decreed by public vote never to let this day go unobserved, but to celebrate the thirteenth day of the twelfth month, which is called Adar in the Syrian language, the day before Mordecai’s day.”

Here like in 1 Maccabees, chapter 7, they will keep this day as a memorial, the day before Mordecai’s Day, the 13th of Adar, as the celebration of this event. However, here it is a public vote. So that Purim is then connected to this event with a clear reference to the Book of Esther, chapter 9, with the mention of Mordecai.