Der Stürmer had a bee in its bonnet about the Jewish holiday of Purim, specifically about what it celebrates: the delivery of the Jews in Persia from Haman, a ruler who wanted to exterminate them. Thanks to Esther, the Jewish wife of the Persian king, Haman's evil plans were thwarted and he was killed together with his ten sons and 75,000 henchmen. According to Der Stürmer this was one in a long line of Jewish attempts at exterminating the Gentiles. The paper ran a whole special edition in May 1934 about the "Jewish plan to murder all Gentiles" and about how Purim was a holiday that celebrates "ritual murder" and on that day Jews go out to waylay and kill innocent Gentiles.

Fast forward to October 16, 1946. On this day Streicher and ten other convicted nazi war criminals are to be hanged. Streicher sees this as a flashback to the killing of Haman and his ten sons, and blames it on the Jews. October 16, by the way, wasn't Purim, that fell on March 25 that year. "Purimfest" is Streicher's shorthand for "The Jews are ritually killing us".

Source: Der Stürmer, Sondernummer (special edition) 1, May 1934. There is a full English translation of this edition online but I am not linking to it as it is on a neo-nazi website.