I always make mention September 24th, 1991 being the single greatest release day of all time
An iconic day. The best rock album of the year ("Nevermind") and the best rap album of the year ("The Low End Theory") released on one day. Plus notable albums from Soundgarden and the Red Hot Chilli Peppers.
EDIT - the other album worth highlighting, not included in the poll, is 2Pac's debut (the brilliantly named "2Pacalypse Now"). If you're only familiar with 2Pac rapping about West Side, drugs and women, etc., this shows a completely different side to him. "Brenda's Got a Baby" and "Part Time Mutha" (which is based upon a Stevie Wonder song) are unflinching stories about teen pregnancy - smart and poignant. Other songs, like "Trapped", "Violent" and especially "Words of Wisdom" are about his frustrations with society, delivered with just the right mix of anger and insight. (The latter was the first rap song I ever heard that spoke critically of Dr. Martin Luther King - obviously a controversial position to take). There's some filler, and the production isn't great (often sounding quite dated), but this is a much more political version of 2Pac, which has largely been forgotten in favour of the West Coast lifestyle he'd soon spend most of his time talking about.