Going to Mikveh on Erev Shabbos:
It is a great Mitzvah to immerse in a Mikveh on Erev Shabbos, and so is the Chabad custom. Immersing before Shabbos draws down the holiness of Shabbos onto one’s soul. Regarding Erev Yom Tov, even one who is not particular to go to Mikveh on Erev Shabbos is obligated to immerse on Erev Yom Tov.
Background of immersion on Erev Shabbos:
- Immersing on Erev Shabbos is recorded in Rishonim [Sefer HaManhig “One who immerses in honor of Shabbos and Yom Tov is not to recite a blessing”]
- And Sifrei Kabalah [Arizal in Shaar Hakavanos brought in Kaf Hachayim 260/6-7 and M”A 285/1; Reishis Chochmah Shaar Hakedusha 7/119; Shelah Miseches Shabbos; Or Tzaddikim 28; Seder Hayom]
- And so is brought in Poskim [Birkeiy Yosef 260/3; Kitzur SHU”A 72/12; Aruch Hashulchan 260/1 and 242/41; M”B 551/95; Chayeh Adam in Zichro Toras Moshe 1/2]
- Immersing before Shabbos is a tradition which was handed down from the Baal Shem Tov. [Sefer Haminhagim p. 50, Vetzaruch Iyun on the reason this is traced to the Baal Shem Tov if so many other previous sources exist for this custom]