Hi there and welcome to OSM!

Temporary features (such as the Eisteddfod Maes) should not be mapped because data consumers that pull OSM data may use it for weeks or months before updating.

https://wiki.openstreetmap.org/wiki/Good_practice#Don't_map_temporary_events_and_temporary_features

You CAN however, add it to OpenHistoricalMap, where mapping is also done by time!

https://www.openhistoricalmap.org/

Happy mapping :)
Ceirios