Square,
Your disrespect is shocking. If the topic doesn’t interest you, don’t post.
This fast is brought in Magen Avraham (או”ח סו”ס תקפה)
נוהגין להתענות עשרים בסיום בכל מלכות פולין
Even for those who don’t fast, it is a day that can be spent in teshuva and introspection. Davening for the end to our suffering in golus and the coming of Moshiach.
See Sefer Hasichos 5751 vol. 2 pg. 611