as part of a 2-game series (see link). Moving it to tonight facilitated a stand-alone Saturday afternoon outdoor game on January 5, and also meant the students would be on campus for the game. Also, it falls between ND's bye last weekend and Michigan's bye this weekend.
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The rest of the Big Ten played this weekend, so it seems like this was our regularly scheduled Big Ten bye in the second half of the season, due to there being seven teams. Michigan played a two-gamer vs. Michigan State.
There were two other full two-game series in the Big Ten January 4th-5th, plus the ND/UM outdoor game. So it seems like that "should" have been the two game home series vs. Michigan, but ND probably asked UM to move the game to make the outdoor game stand alone, plus you had the students gone and campus still in recovery mode from all things Winter Classic.