If you've been watching Game of Thrones for years, then you know by now that the seasons tend to last for literal years in Westeros. The Starks may have always been talking about Winter coming, and now that Jon and Sansa have seen the rare white raven, you can't help but wonder, what does the white raven mean on Game of Thrones? It likely means that Winter is no longer coming, but has finally arrived. When we see the white raven in Sunday's episode of Game of Thrones, it's in Winterfell, just as the snowfall is getting thicker and thicker. Basically, the white raven signifies that Winter is here, which also lends to the idea that the White Walker war will come soon.
What's that, you say? We just had an insane and all out Battle of the Bastards, so how could we possibly handle another Westeros war so soon after? Since the Night King touched Bran, effectively marking him, he's likely to go on the hunt for him, and yes, Bran is now officially the Three-Eyed Raven, but that doesn't mean that the White Walkers aren't going to be coming for him. That's the war we will undoubtedly see in either Season 7 or Season 8 of Game of Thrones.
So if the white raven means Winter is here, then we can assume that the season will last for the remainder of the show, making it possible for a Westeros vs. White Walker battle to ensue at some point. In the books, the sighting of a white raven means that Winter is coming or, at the very least, that the current season is coming to an end. It's probably no coincidence that the white raven comes from the Citadel, where Samwell Tarly has just arrived to get his learning on and become a maester.
But we aren't going to be following Samwell's education for the Hell of it, because when is any story line on Game of Thrones simply gratuitous? Instead, though, we can hope/pray/consider that Samwell arriving at the Citadel and the white ravel being sent to Winterfell at almost the same time means that Samwell is going to be learning things of importance at the Citadel. You know, to make his way back to Castle Black and help Jon when the White Walkers come. Because yes, Winter is here, and the White Walkers are going to come. Even if they're the last "big bad" of the series, Winter will usher in that war.