Health and Sports day was first introduced in 1964 for the first olympics games in Japan and October was chose so that they would miss the rainy season, good idea!
Health and sports day was introduced to encourage more sports but also to be both mentally and physically healthy, schools and businesses also usually chose to use this day for there school or business sports day , so it really does involve everyone in the country. Schools and other business usually choose to do athletic type of games such as sprints ,long jumps ,three legged races and loads of others. The festival starts rather early at 8:30am OUCh ! however they include a lunch break and during this there may be some dancing to watch.
It all sounds super fun to me !