Thank you Toritoribe-san. So if I understand correctly, a sentence like クリスマスは12月25日になる wouldn't make sense, because the date of Christmas is just fixed and is never the result of something else, right?
I suppose an exception could be someone doing historical research to find out why December 25th was chosen, who could say クリスマスが12月25日になりました。Am I on the right track?