A-Day was the catastrophic incident that begins the story of Marvel's Avengers. Occurring five years before current events, what was meant to be a day of celebration dubbed Avengers Day turned into a terrorist attack that destroyed San Francisco and subsequently gave people all around the area inhuman abilities.
Record of Events
On the anniversary of their foundation, the five original Avengers (Captain America, Iron Man, Black Widow, Thor, and Hulk) were unveiling their new headquarters in San Francisco. During the event, they also unvieled their new helicarrier, the Chimera, which was powered by the experimental Terrigen energy crystal. Kamala Kahn was enjoying the event and met Iron-Man, Captain America, Bruce Banner, Black Widow and Thor.
The celebration turned to tragedy though when an explosion rocked the Golden Gate Bridge. So, instead of enjoying "fan clubs and funnel cakes," the Avengers mobilized to combat masked terrorists on the bridge. While they were distracted, their helicarrier engaged its auto-pilot because someone initiated its security protocols. Once the ship was airborne, Captain America boarded it to investigate, but quickly realized its energy source was overloaded and ordered an immediate evacuation of all civilians on board. The ship then exploded in a huge shockwave, and the Chimera crashed into the bay.
Though they rushed in to save the day and the explosion was a mystery even to themselves, the Avengers were ultimately blamed for the event. The news said they "paraded dangerous, untested technology causing damage on an unprecedented scale." Captain America was presumed dead among thousands of other innocents. Hundreds more fell sick, encased in strange cocoons and later emerged with dangerous, otherworldly powers.