Ed: “Search engines mostly say Monday 8th July”
Do they actually say Monday, or mention a year? 8th July was a Saturday in 2023 so I suspect the search engines are finding last year’s information. Last year’s Cardiff’s WNBR was Sat 8th July 2023, according to a flicker report which quotes that date in full.
In any case, a web search would mostly find pages that regurgitate information on the two WNBR pages you mention. But there are two press contacts at https://wiki.worldnakedbikeride.org/wiki/Cardiff#Press_contact that should presumably be able to give you a definitive answer.