What is the earliest a Christmas Mass can be scheduled on December 24th?
Consistent with other “anticipated Masses,” I believe that would be 4 PM. Unlike the Easter Vigil, I don’t believe there is any requirement to wait until dark, 8 PM or so - and certainly in many places it’s already pretty dark by 4-5 anyway.
It seems to depend on the place. 4pm is usually considered appropriate though in some places the bishops have chosen an earlier time.
There’s a long thread from the past week or so here: forums.catholic.com/showthread.php?t=927172
Christmas is actually one of the rare times when there actually really is a “Vigil” Mass (as opposed an evening Mass that is identical to the Mass on the day itself and is often mis-identified as a “Vigil Mass” when it fact it’s the same Mass as the following day). In fact there are several possible Masses for Christmas, all with their distinct readings and propers:
The Vigil Mass the previous evening (ad Missam in vigilia in the Graduale Romanum)
The Midnight Mass (ad Missam in nocte)
The Mass at daybreak (ad Missam in aurora)
Daytime Mass (ad Missam in die)
The earliest one would be the Vigil Mass. It could start any time after First Vespers which would typically be 4 pm at the earliest.
Does the Midnight Mass constitute a Vigil Mass or the regular, on-the-day-of Christmas Mass?
“Midnight” Mass is actually Mass during the night. You can see the different sets of readings here: usccb.org/bible/readings/122514.cfm
The ordo (at least the one for my diocese) says of the Nativity Vigil Mass: This Mass may be used on the evening of December 24, either before or after Evening Prayer I of the solemnity
Which, of course, is echoing the *General Norms for the Liturgical Year and the Calendar *#34.
Neither, it has its own distinct readings and propers.
It’s the same everywhere.
The earliest time for a Christmas Mass (since that’s what you asked about) is the evening (vespere) of December 24.
The text entitled “Vigil Mass of Christmas” may be used on December 24 (earlier than evening), but unless it’s in the evening, it does not fulfill the Christmas obligation as defined in canon 1248.