PL : Deep dark, the postgame ! Explanation in body text.
What an adventure, huh ? You saved an entire island from being destroyed by a tiny ghost. But, it wouldn't be a pokémon game without some content after the main story, right ? The postgame's goal is to use the 4 days to answer the unanswered questions, give you more content, and just have a bit more fun... Man, it's hard to let this series finish.
Day 1 (today) : After the story
Day 2 : One more battle
Day 3 : Festival
Day 4 : Last goodbye
(Note : I had already planned a postgame before the series, I didn't make it up today because I wanted to continue.)
As for what you'll be doing today... It's really up to you. You're the champion, there's no one to fight against. Maybe you want to bond with marshadow. Maybe you want to spend time with your team before the goodbye. Maybe you wanna visit the shop for some reason. You're free to go wherever you want.