It wasn’t made up by white people, Nineteenth has been around since the first anniversary of Union troops marching into Galveston. It isn’t a national holiday because it has much more localized roots, based not on the end of slavery in the US but specifically the end of slavery just in Texas.
But celebrations of it took place in many places in the US, as people moved around and took the Texas tradition with them.
I grew up in Texas. Went to Texas public schools 1st thru 12th grade.
I never heard of it.....