Is December a bad time to submit manuscripts? Or are editors too busy with end of the year stuff? Would they be more or less likely to give a manuscript their full attention during the holidays?
The number one secret to know about editors and publishers is we are people too. We have parties and gift giving and other merry-making to do, just like everyone else. So yes, we say go ahead and submit in December, but chances are it's going to take a little longer for us to get around to reading it. And one good thing about LDS publishers, we're rarely working with an after-office-party hangover. So submit whenever you want with full confidence that when we do (finally) get to your manuscript, it will get our full attention--just as it would during the rest of the year.
[P.S. You've just stumbled upon the REAL reason I'm running a contest in December. I'm hoping your stories will disguise the fact that I'm too busy ho-ho-ho-ing to write serious posts. And speaking of your stories, where are they??]