The epic story of the Purim story tells of how the wicked Haman wanted to kill, to destroy, and totally annihilate the Jewish people. Unfortunately, a theme that constantly had repeated itself through the ages. What was unique about Haman’s final solution was that he wanted to accomplish that feat all in one day.
He cast lots to determine the most opportune month to carry out his murderous plan. The month that was chosen, was Adar. This was repeated many times in order to confirm that it was not just happenstance. If you pick a month out of a hat, it has to land on one of them. Repeatedly, the month that came up was Adar, the 12th month of the Jewish calendar.
Haman was very excited that he had discovered the month that was so auspicious for him to carry out his evil plan. He felt that it was truly a good month because that is when Moshe Rabbeinu died and he felt that it was very auspicious He then went about choosing the day. The most obvious choice would have been the first day of the month. Haman’s hatred to the Jews was so strong that it is hard to understand why he would not have wanted to fulfill his desire at the earliest time. But in truth, the day that he chose was the 13th.
The question is why. If it wasn’t the first of the month, the next logical day would be the seventh, the day that Moshe Rabbeinu died. That was the reason that he was so thrilled when the lots landed on Adar so why not on the 7th?
The answer is that the 7th was indeed the best day, but Haman was afraid that the merit of Moshe would remain in this world for the seven days of mourning, following his death. Hence he waited until that period had passed and then was confident that he would succeed.
It might be for this reason that 13 is looked upon as an unlucky number. Many buildings don’t have a 13th floor and other silly superstitions are based on the number 13. See https://en.wikipedia.org/wiki/Friday_the_13th for more info on this topic
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