The Battle Barry every day glitch allows the player to battle their rival (Barry) at the Fight Area every day. It only exists in Pokémon Diamond and Pearl, and it does not occur in Pokémon Platinum.
The player can battle their rival in the Fight Area every day of the week simply by saving after the battle and not doing anything until the next day. The midnight exploit can also be used. Next time the game is started up, or after it becomes midnight (start of Monday) from within the game, the player's rival will still be standing there, ready to battle again.
Normally, it is only possible to battle Barry here on a Saturday or Sunday as reloading the area through a warp, Fly or the Explorer Kit causes Barry to disappear after Sunday has passed.
The glitch does not occur in Pokémon Platinum (where Barry is found at the Survival Area instead of the Fight Area) because Barry is programmed to give a random message instead of battling the player outside of Saturdays and Sundays. He will also no longer disappear outside of Saturdays and Sundays after the Survival Area is reloaded in one of the ways described above.