this shabbos is shabbos (duh) yom kippur, which is called shabbos shabboson, and in a shmittah year, which is also called shabbos. Is there any significance to this, and when was the last time this happened?
shabbos on Yom kippur isnt rare (in fact it is the most likely day though not by far) it occurs slightly less than 1/3 of the time. Shemitah as You know is one in 7 years so about every 3-4 shemitah or 21-28 years you can expect your “3 in 1 Shabbos”