There are four DLCs available to play. Final Fantasy 15And even that is a choice. But they all play a part in the overall story and occur in specific chapters. Each party member has moments in the game when they are separated from the others. And during that time each DLC will take place.
Each DLC gives a better perspective on how and why characters return to change after being apart. While also providing a unique play style as you learn to play as someone other than Noctis, here are the best times to play each DLC.

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When should you play each DLC in Final Fantasy 15?
Each DLC takes place at a specific time in the story. Although you can choose to play anytime after that. If you want to focus on the main story of Noctis for a bit longer. That being said, playing the DLC before it takes place in the story will definitely lead to spoilers. Especially with the final DLC that will be included in this article.
Each DLC is around three or four hours long with boss fights and time trials options after you finish the main story.
When Gladiolus was born
After the events of Chapter 6, Gladiolus leaves the party and Takes up most of Chapter 7.– During the time the events of Episode Gladiolus take place, you can choose to play this DLC before or after he rejoins the party. As long as you play and defeat the boss after he leaves the party.
When Ignis was born
Ignis episode Happens at the end of Chapter 9. After Noctis fights Leviathan and falls unconscious. Although he doesn't leave the party the same way Gladiolus does, the DLC's events take place between Chapters 9 and 10, with Noctis waking up long after the DLC's events end, so you You can choose to play at any time after defeating Leviathan.
When Episode Prompto Happened
at The beginning of Episode Prompto takes place during Chapter 12. Once Prompto is knocked off the train, the DLC's activities continue until after Chapter 13, when he rejoins the group. and should be played after he rejoins at the end of Chapter 13.
When will the Ardyn episode take place?
The events in this DLC take place many years before the events of the game. But everything about Ardyn is intentionally left a mystery until the end of the game. It is recommended that you beat the game first. And learn all the surprises about Ardyn before learning his backstory. Because it would ruin the surprises that are commonly talked about in DLC.