Best Answer - Chosen by Voters
The first key point would have to be when Laura's helping out Pa in the fields, and he discovers the muskrat's nest, built out of mud. He tells Laura that it's going to be a hard long winter and he can tell this by how thick the walls of the muskrat's nest is. The muskrats know, instinctively, that a long winter is coming and so they built their nests thicker and sturdier so as to ward off the long months of cold and snow.
Another key point would be the old Indian that comes into the drugstore where Pa and some of the other townsfolks are shopping. He tells them that every 7 years there's a long hard winter, and that the 7th of those 7 (or every 49 years) is the worst winter of all; there will be 7 months of blizzards. This year (the year of "The Long Winter") is the 49th year so it will be the worst. Pa takes this information seriously and moves his family out of town and into the town of DeSmet.
Another key point would be when the blizzards get so bad that the trains can't get into DeSmet and Pa learns that there will be no more trains (and no more food coming in) until the long winter is over. The Ingalls had to learn to survive on such meager food as potatoes and brown bread (wheat kernels were ground into flour by hand using the tiny coffee grinder) for months.
Eventually even that minor amount of food runs out and the Ingalls, along with the rest of the town, are slowly starving to death with no hope in sight. Then Almanzo Wilder and his friend Cap Garland hear about a man living on his own, miles out of town, who has a very large amount of wheat stored in bags. They decide to risk the terribly dangerous journey through blizzards to buy the wheat from the man. They reach the man days later and haggle over the price of the wheat. The man at first refused to sell them the wheat, then tried to make a profit on it, but when Cap Garland angrily tells the man that people will starve to death without his wheat the man becomes ashamed and sells the boys the wheat at the same price it cost him. The boys load up the many bags of wheat on their sled and head for DeSmet. Days later they reach the town and everyone has enough wheat (rationed out very carefully) to last them through the long winter.
Finally, months later in May, the Long Winter is over. The trains were running again and the Ingalls family received a Christmas barrel from their good friend Reverend Alden. In it were newspapers, clothing and good food including a turkey, still frozen. So finally the Ingalls family was able to sit down with their friends the Boasts and have their Christmas dinner in May.