If you're jumping back into The Witcher 3, you might want to know the best time to start Blood and Wine. Since this is the second expansion that takes place canonically after the events of the base game's story, you should start Blood and Wine last, once you've done everything in the main game and Hearts of Stone. We'll elaborate more on when to start Blood and Wine below!
You should only start Blood and Wine after you've completed the base game's story and Hearts of Stone, as Blood and Wine canonically takes place after the events of the story. If you play Blood and Wine between the main story, it will feel oddly disjointed and will potentially hinder the narrative experience of both The Wild Hunt and Blood and Wine.
Without going into heavy spoiler territory, The Wild Hunt is all about finding Ciri before the Wild Hunt does, so for Geralt to take a detour to enjoy the sun, some wine, and the sights of Toussaint to hunt vampires wouldn't really make a lot of narrative sense when the fate of the world (and his daughter) hangs in the balance.
Blood and Wine can be started with "Envoys, Wineboys", a quest found with the "Appeals to Sir Geralt of Rivia, the Bridge his Crest" contract on noticeboards in Velen, but only after completing "A Poet Under Pressure". This starting quest is Level 34, and some of the missions in Blood and Wine have a recommended level of 50, so you'd be making a grave mistake jumping into the expansion before then!
It's canon that Blood and Wine takes place after the events of The Witcher 3: Wild Hunt, so you should only start with the expansion content when you've experienced all that the base game has to offer. You can treat Blood and Wine as the epilogue to Geralt's story, or perhaps even a whole new chapter.
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