Hi, may I ask for an advice?

which is correct

It will snow on Christmas.
It will be snowing on Christmas.

is there any rule for it?

thank you vey much

There is very little difference between these two phrases. The second emphasises the action a little more.

Note the preposition usage:
It will snow at Christmas/It will be snowing at Christmas.
It will snow on Christmas day/It will be snowing on Christmas day.