This is the time of year when rectory phones begin to ring with the great question titled above. Even though it might sound funny, it's a darn good question, as evidenced by this Mass schedule from one of the places I celebrate these days:
After all, the rubric only says, "at night," so at the end of December in the northern hemisphere, 7:00 pm certainly qualifies.
At the parish where I was most recently assigned, the pastor moved the Christmas Mass in nocte to 10 pm, taking his lead from no less a brother pastor than His Holiness Benedict XVI.
Some of the parishioners were horrified. Knowing that I can sometimes have traditionalist leanings, they tried to make an ally out of me in their campaign to restore the Mass in nocte at midnight. I tried to make a deal with them: I would support their efforts, if they would support me in agitating for the Easter Vigil to be celebrated also at the most traditional time, i.e. such that it would end right before dawn on Easter Sunday. They did not accept my offer.