Trump did not make Juneteenth famous by initially choosing to hold his first campaign rally in Tulsa on the day.
However, as opposed to his claim, Trump is not responsible for making Juneteenth famous. Juneteeth goes back to 1865 and is the oldest national holiday in the US. It commemorates the ending of slavery in the country. According to a New York Times report, Trump and some members' campaign teams did not know why the date was meaningful.
Therefore, Trump did not make Juneteenth famous- it is the oldest US holiday, one that Trump and many members of his team were unaware of and claimed to celebrate through his rally in Tulsa.