It is very difficult for it to be ‘too cold for snow’. I’ve seen several people state this modern myth today on Facebook and it was also said at my bus stop. On Earth, it is entirely possible for it to snow far below 0°C. It does snow in many sub-zero places everyone is quite familiar with, including the poles and on the top of mountains.
Snowflakes form when droplets of water are pushed upward by air currents and freeze onto the surface of a microscopic dust grains in the air. The ice structures grow outward, collecting more water from more droplets in the air. The snowflakes grow as they rise into cooler air and eventually become large and heavy enough that they fall. If it’s cold enough all the way down, they will still be ice crystals when they reach the surface – maybe near you. Boom – it’s snowing!
So long as there is moisture in the air and the temperature is cold enough to freeze the snowflakes then snow can form. One reason the ‘too cold to snow’ misconception might occur is that cold conditions are often very dry. There is usually less moisture in the air when it’s really cold. For example, the lovely blue Winter skies that many of us enjoy are unlikely to form snow if there aren’t even clouds!
As an aside, the shape of each snowflake is determined by the exact shape of the dust grain, the precise conditions that condensed the droplet, the way the temperature changed through the snowflakes journey upward and then down to the ground. There are so many factors involved in this that is very unlikely two snowflakes would form in the same way and hence have the same shape. So the idea that snowflakes are all unique probably is true.