No, it's not Westeros where winter lasts a long time or when the boring Starks give inaccurate weather predictions for years.
It was 1816, the following year after Mount Tambora erupted. Mount Tambora stood at 4300 meters high (14107 feet), located in Sumbawa, Lesser Sunda Island, Indonesia.
It was scaled at 7 VEI (Volcanic Explosivity Index), spewing over 150 cubic km (around 36 cubic miles) volume of materials; ash, pumice, rock, sulfur etc. triggering tsunamis, with casualties expected around 71000 to 91000 deaths in Indonesia alone. Direct casualties were estimated at 10000, with the follow-up famine and disease tolls from longer-term impact.
The explosion of Mount Tambora was so powerful, and the materials mixed with atmospheric gas prevented substantial amount of sunlight from reaching the earth, it lowered the earth's temperature significantly around 1 to 3 Celsius degrees (note: various numbers from different sources, perhaps it was based on geographical impact differences). If it doesn't sound like a lot, consider this: crops failures, famine ensued, and there was no summer in the following year. The impact reached Western Europe and eastern North America that experienced sporadic periods of heavy snow and killing frost. It caused food shortages all around Northern Hemisphere. 1816 was known as the year without summer.
The explosion created a caldera with a diameter of more than 6 km (3.7 miles) and 600-1250 meters (2000 to 4000 feet) deep. Mount Tambora was reduced to 2850meter high.
These are some recorded impacts in Indonesia.
Source: Idntimes.com
