Game of Thrones has been teasing winter's arrival since its very first episode, which was aptly titled "Winter Is Coming." The words of House Stark are foreboding, yes, but what does winter even mean on Game of Thrones? As of Sunday night's Season 6 finale, titled "The Winds of Winter," winter is finally here — so it's about time we start preparing for the darkness to come.

Before we get into the logistics of a decade-long winter, it's important to remember that throughout the entirety of the series Westeros has been enjoying a nice, long summer of peace and plenty. In fact, at the start of Game of Thrones, Ned Stark states that summer had already lasted nine years — the longest summer on record — and that, yes, "winter is coming." In George R.R. Martin's A Clash of Kings, the arrival of a white raven announced the end of summer and the beginning of autumn.

So when Jon Snow and Sansa Stark receive a white raven from the Citadel to announce the arrival of winter, think of it as a formal announcement of the changing seasons. No one knows for certain how long this winter will last, but seasons in Westeros are rather unpredictable. However, conventional wisdom in the North (a.k.a. Old Nan) suggested that this winter should be roughly as long as the previous season. In other words, a long summer means an unbearably harsh winter.

This is cause for alarm for many reasons, but mainly because Westeros isn't prepared for a long winter. In Season 1, King Robert's small council discussed preparations for an exceptionally long winter, but as with most things under Robert's rule, nothing really came to fruition. With all of the changes since — Westeros has seen two kings and one newly-appointed queen since Robert's death — there's no way the Seven Kingdoms are prepared for winter. For the past several years, the kingdoms have been plagued by the War of the Five Kings, which has left few lords prepared for the winter. Keeping men fed and warm will prove to be a challenge for southern forces such as the Lannisters and Daenerys's Dothraki army. Simply put, Westeros is extremely vulnerable right now.

Snow will fall as south as Highgarden and King's Landing. (It's stated in the books that only the southern tip of Dorne is spared from winter's reign, as snow rarely falls in the city of Sunspear. So when that inevitably happens, you know it's bad.) The nights will be long and cold. Some even believe that this forthcoming winter will be as bad as the Long Night.

According to Westerosi legend, the White Walkers first invaded Westeros thousands of years ago during a winter that lasted an entire generation, pushing humanity to the brink of extinction. This is known as the Long Night. Old Nan described the Long Night to young Bran in Season 1, telling him, "Oh my sweet summer child, what do you know about fear? Fear is for the winter, when the snows fall a hundred feet deep. Fear is for the long night, when the sun hides for years and children are born and live and die, all in darkness. That is the time for fear, my little lord, when the White Walkers move through the woods."

"Thousands of years ago there came a night that lasted a generation," she continued. "Kings froze to death in their castles, same as the shepherds in their huts. And women smothered their babies rather than see them starve, and wept and felt the tears freeze on their cheeks."

Legend has it the children of the forest and the First Men eventually teamed up to eventually stop the White Walkers from destroying mankind, and with the help of the Last Hero — Azor Ahai, otherwise know as Melisandre's Prince Who Was Promised — they pushed the wights back into the northern Lands of Always Winter. Then Bran the Builder, the founder of House Stark, built the Wall with the children of the forest to permanently keep the White Walkers at bay.

It sounds familiar, doesn't it? We know the White Walkers are pushing south in search of Bran Stark, who finally made it to the Wall in the Season 6 finale. Once they destroy the Wall, it's game over for the Night's Watch and quite possibly mankind as we know it. That is, unless the Mother of Dragons and Jon Snow, the Prince Who Was Promised incarnate, come up with a better plan.

Regardless of what happens, winter's immediate arrival ensures that Westeros will be a very different place when Game of Thrones returns next year.