What did the ringing of bells throughout the US's National Park Service areas on 19 August 2019 mark?

Question

On 31 July 2021 what was one event held across the City of London to mark its reopening as the lockdowns of COVID-19 eased?

Answer 1

The 400th anniversary of the landing of Africans traded as slaves in English-occupied North America
The landing was at Point Comfort in Hampton, Virginia, and the location is now part of the Fort Monroe National Monument, a unit of the National Park system. The Act which established the Service was passed in 1916.