You can check out the official announcement for The Guardians of the Galaxy Holiday Special above, thanks to the official Marvel Studios Twitter account.
The Guardians of the Galaxy Holiday Special will film simultaneously with Guardians of the Galaxy Vol. 3.
As for what The Guardians of the Galaxy Holiday Special will be all about, that is unclear, other than the holiday aspect of it.
Guardians of the Galaxy Holiday Special is officially happening at Disney+ with James Gunn on board to direct.