וַיִּרְא֥וּ אַנְשֵֽׁי־אַשְׁדּ֖וֹד כִּֽי־כֵ֑ן וְאָמְר֗וּ לֹֽא־יֵשֵׁ֞ב אֲר֨וֹן אֱלֹהֵ֤י יִשְׂרָאֵל֙ עִמָּ֔נוּ כִּֽי־קָשְׁתָ֤ה יָדוֹ֙ עָלֵ֔ינוּ וְעַ֖ל דָּג֥וֹן אֱלֹהֵֽינוּ׃
And when the men of Ashdod saw that it was so, they said: ‘The ark of the God of Israel shall not abide with us; for His hand is sore upon us, and upon Dagon our god.’
Megillat Ta'anit
This is what is written “And David conquered the stronghold of Zion which is the city of David.” (Shmuel I 5:7) this is the place of the karaites (?) right now. Since they were troubling the people of Jerusalem and they were unable to go out or in except at night, they declared the day they were driven out as a holiday.