How exactly do you get the snowdown chests?

Right now there is a timer on my client which is like a time bomb, Saying Today's snowdown chest: MistleToe! Contain's heart themed skin shards.

Do you get it the old fashion way by getting an S rank with a champion you haven't gotten before, And can you earn them if you have no chests available, Or do you just play and play and play until you get one, Like fragment keys but you only have a chance to get them if you win?