Why does your freezer mist when opening door?

When the freezer door is closed the freezer freezes all the water vapour that is in the air inside the freezer compartment. First the invisible molecules of water vapour in the air condense into tiny water droplets, making a mist. Then those droplets freeze solid at 0oC.

When the freezer door is opened, the new air from outside has invisible water vapour in it, so the coldness from inside the freezer makes the invisible molecules in the new air condense, thus forming a mist of tiny water droplets suspended in the air.

So opening the freezer lets the cold air from inside the freezer cool and condense the water molecules that are in the air just outside the freezer. The result is the mist that you see, a swirling mini-cloud of tiny water droplets suspended in the cooled air.

Some of this mist will go on to the cold surfaces inside the freezer compartment, where they will freeze solid when the door is closed.

Most people don't want too much ice in the freezer compartment, so it is best to not open the freezer door too often. And then make sure that it is closed quickly afterwards, otherwise a lot of ice will build up in the freezer compartment and it will become more difficult to remove the frozen food.