Which groups of Jews observe the monthly holiday known as "Yom Kippur Katan," in which one dedicates the day before Rosh Chodesh to repentance, often including fasting and selichot?
In what ways do different groups interpret the custom? Is it considered a firm requirement, a fixed minhag, or simply an optional extra tool for spiritual improvement? May one do it some months but not others?
In Yerushalyaim it is common to find minyanim saying it. Furthermore, it is the official practice of the Mir yeshiva to say Yom Kippur Koton.
Although the minhag is not widespread being that it is something that was practiced by exceedingly righteous as its source is in kabbala
The minhag is kept by Yehudei Polin. In the Machzor Polin MiKol HaShonnoh, it says that on Yom Kippur Koton, people should fast but that even if you don't fast you still say the selichos at Minchoh. Also, it says if you fast, then you put on tallis and tefillin at Minchoh. See http://www.kayj.net/en/forum/nusach/452-yom-kippur-koton-a-minhag-ashkenaz#667 for the Ashkenazi viewpoint.
The selichos are said in the GGBH.
