Juneteenth is a term applied to June 19th, the end of slavery in the United States. Everyone should know the date, primarily the President of the United States.
Assuming that the current POTUS was ignorant of the date, those around him would know. But the date was selected. Selected and promulgated.
This selection of the date was done during the protests of racism in America. During the upsurge of Black Lives Matter.
The date was deliberately selected during this time to inform the Base of support for the current POTUS that he as they, is a White Supremacist who does not agree that Black Lives Matter.
Although he changed the date late on the 12th of June, enough time was allowed between the announcement and the change to insure his Base supporters that he agrees with them.
He also chose Tulsa, Oklahoma for a specific reason.
As most people are ignorant of history, (due to the deliberate suppression) there are those unaware of what happened in Tulsa in 1921.
There was a massacre of African Americans, a burning of a neighbourhood, even the dropping of incendiary devices. It is the worst incident of racial violence in American History.
Every White Supremacist knows about Tulsa, considers it a great victory in the race war. This is why the POTUS chose that city. He chose it as proof to his Base that all the protests will not change the racist standards of America.