Last year we addressed the question "What time can Midnight Mass begin?," which is usually a novus ordo concern, but can sometimes find its way into traditional Latin Mass parishes.
The answer, using the 1962 calendar and its rubrics/liturgical law, is of course no earlier than Midnight. The liturgical law specifying the calendar day timeline for the use of traditional Latin Mass propers is found in the very beginning of the rubrics governing the 1962 liturgy, under part one ("General Rubrics"), chapter two, number four:
4. Dies liturgicus est dies sanctificatus actionibus liturgicis, praesertim Sacrificio eucharistico et publica Ecclesiae prece, id est Officio divino; et decurrit a media nocte ad mediam noctem.
4. The liturgical day is the day sanctified by the liturgical rites, especially the eucharistic Sacrifice and the public prayer of the Church, that is the divine Office; it runs from midnight to midnight.
The rubrics go on to explain more solemn days have an Office that starts the evening before. But it is very clear the rubric is talking about the Divine Office -- that is, First Vespers -- not the celebration of Mass.
More recently, the Pontifical Commission "Ecclesia Dei" (PCED) was asked by a priest about anticipating the Mass propers (that is, using the propers of a day of precept that has not yet started). Rorate covered the answer here. The priest used an example of a Saturday evening traditional Latin Mass, asking if it was licit to use Sunday's Mass propers, to which the Vatican responded "the Mass formulary used should be that permitted on a Saturday."
While it is true the obligation to hear Mass on Christmas Day can be fulfilled the night before -- the result of a 1967 invention by Paul VI, codified in the post-Vatican II code of canon law, it is not true that the first Mass of Christmas propers are permitted any time before Midnight. A Mass starting at 10 p.m. or 11 p.m. on Christmas Eve, for example, must use the Christmas Eve propers -- with violet vestments, no Gloria, no Alleluia, no Credo and the common (or Advent) preface is said/sung. The rubrics are clear enough on this point, but the new PCED letter should put to rest any existing uncertainty.
The traditional Latin Mass follows the liturgical law in place during 1962 (not 1967, or later). For anyone still on the fence concerning this point, see #28 of Universae Ecclesiae:
28. Furthermore, by virtue of its character of special law, within its own area, the Motu Proprio Summorum Pontificum derogates from those provisions of law, connected with the sacred Rites, promulgated from 1962 onwards and incompatible with the rubrics of the liturgical books in effect in 1962.
