Minecraft how to get in winter mode?

Find the snow biome. It always has snow there. I believe it's a rare occurence but it's possible to snow in other biomes as well. But the snow biome is literally the only area where it almost always snows.

http://minecraft.gamepedia.com/...com/Biomes

more info about the biomes.

Go to a snow biome OR find a really tall mountain in minecraft 1.7.2 (Or higher. 1.7.4) it snows on top of really tall mountains.