Northern hemisphere late fall through early spring, which means from roughly November through April, with November and April being warm in the south and west but still mostly cold in the north and northeast.
Depends on what you mean by "cold" of course, but the coldest temps tend to occur from about early-mid-January through end of February. Cold temps in Florida or California would be the warmth of late spring and mid-fall up north. It is a big country, after all (and I pretty much ignored Alaska and Hawaii).
Where I live in the northeast, we tend to see the first snowflakes at about the end of October even if we might not see snow staying on the ground until late December (usually before of course), and the last snow is usually around about mid-April. Folks down south are already into flowers and flowering trees before we even start to see the earliest crocus blooms up this way, usually, never mind tulips or even leaves on trees.