Stratospheric Reflection events, which are triggered by Pacific trough regimes, trigger a transition to AK ridging and colder patterns across the CONUS. They are more common in +QBO seasons (30/44 since 1980), which is maybe what snowman was getting at....but I am going against the grain a bit in feeling as though we get one due to the prevlence of them in my analogs.
Usually very warm a few days prior to a few days after the start, but it begins turning colder quickly....so I guess you could say mid January warmth. Early to mid-January, anyway.