I'm going to try very hard to get a PS for Christmas.:
I think you'll increase your chances if you don't have a set time in mind. I plan to ask for the latest time they have for my party size. That way we'll get a chance to do a few rides before breakfast.
If you don't get a PS the first time, call back. I'm sure some people will others calling to help get a PS. If they get several, they'll call back to release the ones they don't want.
We did this last year and we had our DU agent try to get it and we tried ourselves too. Both of us got it!
I think you have more of a chance to get it for Christmas than other days. My reasoning is most days the only ones calling at 7:00am are trying for Cindys so it fills up fast. On the 90 day mark last year, reports here on the Dis were that people were calling at 7:00am to get their Christmas dining booked, no matter where they wanted to eat. Everyone was booking ASAP for that day, all meals all restaurants, so Cindys didnt necessarily fill up as fast as it usually does. Of coure I dont know this for sure, but its my theory.
Good luck! Christmas morning in the castle was very magical.