A couple more clarifications:
1. It may be too early to buy tickets for most shows for next December. Most do not sell them this far in advance.
2. It's definitely much too early find any discount offers for the end of the year. You'll have to wait several months (probably October) to find any.

This past year anyway, most of the advance sale discounts (broadwaybox, playbill, theatermania, hitshowclub) were only good until 12/21 and after New Years. Christmas week is a tough time for discounts.