To the contrary. Make December 18 the date. It is now a national holiday. Not a black or white holiday. A national one. Like the whole nation which includes we white folks. If a few people in Texas or elsewhere, or thousands or them, got drunk and partied on June 19 for the last 100 years and still do, then good for them, they can still do it.
But, alleviate the ignorance of anyone who believes that slavery ended on June 19, 1865 by setting the date on when slavery actually ended. Or if you choose not to do that, set it for the second Tuesday of August. No problem. Don’t set a pretend date.