Ezra, head of the Sanhedrin and the leader of the Jewish people at the time of the building of the Second Temple, made an historic address to a three-day assemblage of Jews in Jerusalem, exhorting them to adhere to the teachings of the Torah and to dissolve their interfaith marriages (the Jewish people were on the verge of complete assimilation at the time, following their 70-year exile in Babylonia).
The Rosh Hashanah ("new year") of Chassidism, marking the liberation of Rabbi Schneur Zalman of Liadi and the subsequent blossoming of Chabad Chassidism, is celebrated for two days, Kislev 19-20. (The Rebbe was released from prison on the 19th, but his full freedom was only obtained late in the evening -- Kislev 20 on the Jewish Calendar.) The two days are celebrated with farbrengens (Chassidic gatherings) and an increased commitment to the ways and teachings of Chassidism. Tachnun (supplication) and similar prayers are omitted. For more information and links, see entries for yesterday Kislev 19.