While the rest of Israel celebrates Purim this Sunday (the 14th of Adar), Jerusalem celebrates on Monday (the 15th of Adar).
Well, the easy answer is "because Jerusalem is a walled city from the time of Joshua."
Which is partially right. Jerusalem was a walled city in the time of Joshua, but the walls we see today were built in the 1500s, in the Ottoman Era. From the early 13th century and until the mid-16th century, Jerusalem was not a walled city at all. And indeed, it was unclear to the Jews of that time when they should celebrate Purim.
Rabbi Eshtori Ha-Parchi of the 14th century tells us that when he came to Israel, he was told that in Jerusalem they celebrated on both the 14th and 15th of Adar, as they were uncertain which one they were obligated to keep. Rabbi Eshtori brings an entire Halachik discussion about what should be done, and adds that he wrote his rabbi, Rabbi Matityah in Bet-Shean, to ask him what he should do.
Rabbi Matityah wrote him back: If I would be in Jerusalem on the 14th of Adar, and they would read the Megillah, I would leave the synagogue. Otherwise they could say about me "The fool walketh in darkness" (Ecclesiastes 2, 14). And the same is true for Tiberias.
Rabbi Eshtori finished by saying that Rabbi Matityah is right.
We don't know what changed the minds of the Jews of Jerusalem, but today there is no doubt - and we celebrate Purim in Jerusalem on the 15th of Adar.
See here for an archive of articles about our history in Israel.
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