It's really entirely up to you. If you submit your Christmas story in July and don't care when it's uploaded, it will get added to the upload queue/website just like any other story in July.
If you want to make sure your holiday story is uploaded time specific to the holiday - just submit it round about that time.
It's impossible to give you a 'lead in' time as it depends on so many factors - how long the story is, how much work it requires, how fast the GE, RL ambushes... But generally speaking, if you want your story to be uploaded during December, you'd have to submit it by December 1st. And stipulate when you send it in if there's a particular date you want it uploaded by and we'll do our best to accommodate you.
LabRat