Since the mid-20th century, the Church has allowed Catholics to attend vigil, or anticipated, Masses for Sundays and holy days of obligation as “a convenience for many of the faithful.”
“Most canon lawyers defer to Venerable Pope Pius XII’s apostolic constitution Christus Dominus (Jan. 6, 1953), which gave 4 p.m. as the earliest time when anticipated Masses may be scheduled,” the bishops said in their 2017 letter.
This means that the Sunday obligation for Dec. 24 can be fulfilled on Sunday or anytime after 4 p.m. on Saturday, Dec. 23, and the Christmas Mass obligation can be fulfilled on Monday or anytime after 4 p.m. on Dec. 24.
In the case of two consecutive days of obligation, as at Christmas this year, the “prevailing view of many canon lawyers is that each obligation must be fulfilled with a separate Mass,” the bishops said.
“Thus, when consecutive obligations occur on Saturday-Sunday or Sunday-Monday, the faithful must attend Mass twice to fulfill two separate obligations.”
According to the bishops, the question of whether such obligations could be fulfilled in one Mass has been raised before by bishops in a “dubium,” which was “answered in the negative by the Sacred Congregation for the Clergy and approved by Blessed Pope Paul VI in 1970.”