The 11th day of both halves of the lunar calendar month is called Ekadashi. Some Vaishnavas will observe fasting on these days according to what their guru has instructed them. There are therefore 24 Ekadashi's during the lunar calendar with two extra during Adhik Maas (also known as Purushottam Maas).
84 has great significance in Pustimarg. Shri Mahuprabhuji sat in 84 places to read the Srimad Bhagavtam; He wrote 84 works; The Brahma Sambandha mantra has 84 syllables; the circumference of Vraj is 84 kos (168 miles); there are 8,400,000 births through which a soul must pass before achieving human birth and He had 84 close disciples.