I never has seen exact definition of the time. Canon 1246 says ecening
Can. 1248 §1. A person who assists at a Mass celebrated anywhere in a Catholic rite either on the feast day itself or in the evening of the preceding day satisfies the obligation of participating in the Mass.
Can. 1248 — § 1. Praecepto de Missa participanda satisfacit qui Missae assistit ubicumque celebratur ritu catholico vel ipso die festo vel vespere diei praecedentis.
Traditionally the Vespers in choir were allowed after 2 PM, also in the traditional office the Matins (of next day) was allowed to be anticipated after 2 PM. This suggest that 2 PM mass legally can be anticipated Mass, celebrated for the following Sunday of Feast and thus fulfilling the obligation.
In our area (North Suburbs of Chicago) there are often 2PM masses in Christmas Eve