Rodeo grounds are at S8 on the map: https://lamanzanilla.info/la_manzanilla_maps/map_of_la_manzanilla.jpg

The rodeo was started by some of the original expats of La Manzanilla to give something back to the community. It has since grown into a bigger enterprise and there isn’t a specific “gringo day” anymore (at least that I know of). However all are welcome to enjoy the festivities.