Set approximately 12 years before the main story starts, before Luffy receives his signature eye scar or the straw hat from Shanks.
Despite their bickering, they spend the day together in Windmill Village, competing in games and discovering a mutual love for freedom and adventure. One Piece Film: Red Episodio 1
"A Pledge for the New Genesis! Luffy and Uta!" Set approximately 12 years before the main story
For the full backstory before watching the movie, viewers should watch the two specific tie-in episodes available on platforms like Crunchyroll : One Piece Film: Red Episodio 1