The celebration begins on some days with Evening Prayer I, but the liturgical day remains midnight to midnight as promulgated in the the last liturgical calendar. It is much easier in the Byzantine Catholic liturgical calendar which, following the practice of the Athenian day, is evening to evening, rather than the Roman day midnight to midnight.
14 FEBRUARY 1969 ** I. The Liturgical Day in General**
3. Each day is made holy through the liturgical celebrations of the people of God, especially through the eucharistic sacrifice and the divine office.
The liturgical day runs from midnight to midnight, but the observance of Sunday and solemnities begins with the evening of the preceding day.
ewtn.com/library/CURIA/CDWLITYR.HTM
From 2005, **Proposition 30,
"Dies Domini"Although Saturday afternoon belongs already to Sunday (First Vespers), and it is permitted to fulfill the Sunday obligation with the pre-festive Mass, it is necessary to remember that it is the day of Sunday in itself which must be sanctified so that there is no “void of God.” Source: http://biblelight.net/star.gif ** The Synod of Bishops on the Eucharist, held in Rome, Oct. 2-23, 2005. __________________