On Kislev 21 of the year 3448 from creation 313 BCE, there occurred the historic meeting between Shimon HaTzaddik and Alexander (‘the Great”) of Macedonia.
The Samarians, bitter enemies of the Jews, had convinced Alexander that the Jews’ refusal to place his image in their Temple was a sign of rebellion against his sovereignty and that the Holy Temple should be destroyed. The Kohen Gadol (“High Priest”) at the time was Shimon HaTzaddik, the last of the “Men of the Great Assembly” who rebuilt the Holy Temple and revitalized Judaism under Ezra. On the 21st of Kislev Alexander marched on Jerusalem at the head of his army;

Shimon, garbed in the vestments of the High Priest and accompanied with a delegation of Jewish dignitaries, went forth to greet him. The two groups walked towards each other all night; at the crack of dawn they met. As Alexander beheld the visage of the High Priest, he dismounted his horse and bowed respectfully; to his men he explained that he often had visions of a similar-looking man leading him into battle. Shimon HaTzaddik brought the emperor to the Holy Temple and explained that Judaism prohibits the display of any graven image; he offered to name all the male children born to priests that year “Alexander” as a demonstration of loyalty to the emperor (which is how “Alexander” became a common Jewish name). The Samarians plot was rebuffed, and Kislev 21 was declared a holiday. (Talmud Yoma 69a)
According to an alternative version, this episode occurred on the 25th of Tevet. Chabad.org