Prior to the story of Purim was there a special yearly reading of Parshas Zachor? At which point in history was it instituted to read Parshas Zachor before Purim?
1It could have been that people would just hear the parsha during the reading of Ki-Seitzei. The problem is, this wouldn't work with a triennial cycle.– YpnypnJan 10, 2014 at 15:18
@Ypnypn: You may be correct. I would love to see a source!– Gershon GoldJan 10, 2014 at 17:37
The Gemara derives via drasha an obligation to read zachor. Accordingly they must have said it even before Purim. They specifically started saying it before Purim sometime before the time of the Mishnah, for the Mishna in Megilah 29a says that we specifically say it before Purim. The reason for this is to combine the themes of destruction of Amalek present in both (Rashi on 30a).
However the original obligation didn't have a specific time attached to it (see Rambam Sefer HaMitzvos: Assei 189; Hil. Melachim 5:5). Rambam (there) fascinatingly cites Shmuel's exhortation to destroy Amalek in the context of this mitzvah. (I Samuel ch. 15). This was definitely not a regular scheduled event. Rambam does write (there) that the mitzvah is to remember Amalek "tamid" which can either be translated as constantly or regularly.
Rav Chaim Kanievsky was asked if the generation that fought with Amalek would be obligated in Parshas Zachor every year ,he answered in the affirmative.