Like Thecoolman5 said, you need to use a particle system. Model your snowflake in another layer (I made a simple one from a flattened icosphere), and make the particles system emit that object. Make a collision for the tree, and turn the stickiness up to 10. In your particle system, turn the gravity all the way off. Put the normal amount under the velocity to around .6, or whatever you like. Obviously make sure it is projecting towards the tree. Also turn the mass way down to .01. Make sure you set the size low enough so the flakes are the actual size of snow. Add some random size and rotation and you’re good!
Just make sure you set the lifetime of your particles way up so they don’t die while they’re on the tree until you want them to.
By the way, you turn the gravity off because if you don’t they will fall off of the tree. That’s why you increase the velocity rather than the gravity. If you do want it to fall off of the tree and kind of flow down it then increase the gravity slightly.
Sorry if you already knew how to do this. I don’t know how experienced you are so I wanted to go into detail.