From the movie : Your name

Comet Tiamat (Tiamato suisei) is a celestial body. It serves as the key point in Your Name. and also cameos in Weathering with You

Overview

Comet Tiamat has an orbital period of 50,200 years. For unknown reasons, every time the comet approaches Earth, a piece of itself always mysteriously break out and crash somewhere around the area that would become Itomori. The last time it appeared, a fragment broken from the comet crashed into the area that would become Itomori during the Upper Palaeolithic, with its crater forming Lake Itomori.

2013 Itomori impact event

On October 4, 2013, a fragment of 40 meters in diameter from the comet hit the town of Itomori at 8:42 pm. Its impact creates a crater 1 km in diameter, with its shockwave spread as far as 5 km, causing an earthquake measuring 4.8 on Richter scale.

In the original timeline, the comet crashes as Itomori people gather at the autumn festival, causing massive casualties. A third of the population, including Mitsuha Miyamizu, are killed. In the second timeline, with the intervention of future Taki Tachibana, mayor Toshiki Miyamizu is convinced to evacuate everyone to the high school grounds and all Itomori inhabitants are saved, with only 104 people injured.