The exact date that Mount Tambora erupted is lost to obscurity since the populace of the area was mostly killed. Those who were far enough for safety yet near enough to note the event didn’t make a priority of recording the date. Best estimates of modern science make the date for 10 April 1815.
The eruption event blew 100 cubic kilometers of pyroclastic trachyandesite into the air, and ripped about 4,000 feet off the top of the caldera—leaving the once 13,000 foot hight peak at about 9,000 feet. The explosion threw enough debris into the air that a mild volcanic winter resulted; it caused crop-killing frosts in North America in June, and dubbed 1816 to be The Year Without a Summer.
In North America, June generally brought average temperatures of about 68-77° Fahrenheit; in June of 1816 there were major snowstorms in Pennsylvania, crops were killed by frost, and rivers remained iced over. Food was scarce, and most items that were harvested were triple the price they had been the year previous. It’s impossible to know how many died from the resulting famine.
At the time, there was no known cause for the freakish weather, though Benjamin Franklin and his experiments with lightning were sometimes blamed for altering nature. Some think that the cold spurred expansion into the American midwest.
But one positive effect of the explosion: all the ash in the upper atmosphere makes for some colorful sunsets.