What devastating event occurs on December 26, 2004 as noted in the memoir, Wave?

Deraniyagala begins her story by describing the morning of the tsunami. She, her family, and her parents were staying at the Yala Safari Beach Hotel on December 26, 2004, where they had spent several days during the Christmas holiday. They planned to return to her parents’ house later that day. With little warning, the tsunami overtook the beachfront area of the national park.