Held annually on the streets, parks and waters of South Boston, the Boston Triathlon returns August 17 & 18 to delight spectators and challenge participants with both sprint and Olympic versions of the run-bike-swim competition. Kids Day takes place August 17, featuring free registration for little ones who can get in on a fun run for ages 1–7, as well as two “splash & dash” swim and run events for age groups 7–10 and 11–15. The adults then take center stage on August 18, leading into a new twist to this year’s contest with the U.S. debut of the Supertri League, which opens its season of professional triathlon races in the Hub as part of the Boston Triathlon. Some of the best athletes in the world—including several Olympic medalists—vie for supremacy as both individuals and teams on August 18 at Carson Beach from 11:30 a.m.–3 p.m.