This is the date when the US Army established authority over Texas and enforced the Emancipation Proclamation.
Slavery was still legal in the states not included in the scope of the Emancipation Proclamation.
If the government had thought up the holiday, that might be a logical choice, but it's a folk festival that came up from below, not something imposed from the top down.
Most slaves were already free by then, so the date wouldn't have much meaning to them.
The Emancipation Proclamation was a presidential proclamation, not a law (which would have required congressional approval), and it could have easily been canceled by President Johnson. It’s just like an executive order.