It has already happened. The event of 22 Tishri (October 7) will now forever be remembered as part of Simkhat Torah. Simkhat Torah of all days, the one day of unbridled joy in the Jewish calendar, must now have its simkha (joy) tempered with a remembrance of evel (mourning).

She-lo ekhod bi-l’vod ‘amad ‘aleinu le-khaloteinu,
Ela she-b’khol dor va-dor ‘omdim ‘aleinu le-khaloteinu.

For not one alone has stood against us to exterminate us,
But from generation to generation they stand against us to exterminate us.

—Hagadah shel-Pesakh