Why did the Soviets invade Finland in the winter?

During World War II's Winter War, the Finns defeated the Red Army despite being vastly outnumbered. Although it had to give up territory to the Soviets, the country ultimately remained independent.

Why did the Soviets invade Finland in the winter?
Finnish soldiers with a Maxim M/32-33 machine gun in the present-day Russian partition of Pítkiarantala. Source: Finnish Military Museum. Credit: Wikipedia