The 20th of Sivan is a longstanding Jewish fast day, initially established by Rabbeinu Tam in 1171 to commemorate various tragedies in Jewish history.
It originally memorialized the Martyrs of Blois, the first victims of blood libel in continental Europe in 1171.
This day commemorates the tragic events during the First Crusade in 1096, where thousands of Jews in France and the Rhineland were massacred. (In response, rabbis crafted the Av Harachamim prayer, recited before Shavuos, to sanctify those who perished and predict divine retribution.) The 20th of Sivan was dedicated as a remembrance day for the Crusades' victims.
Following the Rindfleisch Massacres in 1298, the 20th of Sivan was further marked as a day of tragedy.
Rabbi Yom Tov Lipmann Heller, the Tosafos Yom Tov, ordained the recitation of selichos on the 20th of Sivan to commemorate the Chmielnicki Pogroms in 1648–49.
Additionally, in memory of the Holocaust and the Hungarian Jews' devastation, Hungarian rabbis decided to observe the 20th of Sivan annually after close to 500,000 Hungarian Jews were deported to Auschwitz-Birkenau in 1944.
Following World War II, proposals emerged to designate it as a Holocaust Memorial Day, yet this suggestion did not gain widespread acceptance.
In 1948, Rav Tzvi Pesach Frank advocated for the day to honor the Jewish Quarter's fall in the Battle for Jerusalem.
Rabbi Shabbatai HaKohen, also known as the Shach (1621–1662), notes that the 20th day of Sivan was selected for mourning these tragedies because it avoids coinciding with Shabbas.
Historically, fasting on the 20th of Sivan was common among Polish Jews until World War II. It is now primarily upheld by specific Chassidic communities with Hungarian roots.
The Magen Avraham supports this custom by stating, "It is customary in the entire Kingdom of Poland to fast on the 20th of Sivan." (Magen Avraham 580:9).
The Mishnah Berurah confirms this practice was prevalent in Poland during his time (580:16).
Presently, most contemporary siddurim omit the selichos for the 20th of Sivan, suggesting that many communities have ceased observing this day.
Sources:
https://www.yeshiva.co/midrash/21441 Rabbi Kaganof's
https://www.chabad.org/library/article_cdo/aid/5548614/jewish/Why-the-Fast-on-20-Sivan.html