The Term "Day Player" is a Union term for the very small speaking parts - just a few Lines - where the Actor is only hired for 1-2 days of a 2-3 Month long movie shoot.

People Mis-use the term from time to time but that is its Origin and Usage

The parts Unusually go to Union Actors but it is the Next step for a Non-Union Actor to be able to join the Union
It takes 1 Principal role (Day Player or better) or 3 Extra roles at Union pay - on Union sets to join

It is up to the Casting Director to decide if they will see a Non-Union Actor for the parts - Usually by Invite only

Non-Union Actors that have Impressed the Casting Director by their Professionalism and Talent on other Projects will get the Invites, High maintenance (PAIN IN THE __ __ __ Actors) will not