Part II-2. Celebrations on Sundays and Weekdays
Celebration of the Eucharist on Sunday
25. Whenever the community gathers to celebrate the eucharist, it shows forth the death and resurrection of the Lord in the hope of his glorious coming. But the Sunday assembly shows this best of all, for this is the day of the week on which the Lord rose from the dead and on which, from apostolic tradition, the paschal mystery is celebrated in the eucharist in a special way. 
In order that the faithful may willingly fulfill the precept to keep this day holy and may understand why the Church calls them together to celebrate the eucharist every Sunday, right from the beginning of their Christian formation it should be set before them and instilled into them that Sunday is the original holyday.  On this day above all, gathered as one, they are to hear the word of God and share in the paschal mystery.
Furthermore, all measures should be encouraged that are designed to make Sunday "a day of joy and freedom from work." 
Celebration of Sunday with the Bishop and in the Parish
26. It is fitting that the sense of ecclesial community, fostered and expressed especially by the shared celebration of Mass on Sunday, should be carefully developed. This applies to assemblies with the bishop, above all in the cathedral church, and to the parish assembly, whose pastor takes the place of the bishop. 
It is of great advantage to promote that active participation of the whole people in the Sunday celebration which is expressed in singing. In fact as far as possible the sung form of celebration should be the first choice. 
Especially on Sundays and holydays the celebrations that take place in other churches and oratories must be coordinated with the celebrations in the parish church so that they contribute to the overall pastoral program. It is indeed advantageous that small, nonclerical, religious communities and other such communities, especially those that work in the parish, take part in the parish Mass on these days.
As to the hours and the number of Masses to be celebrated in parishes, the convenience of the parish community must be kept in mind and the number of Masses not so multiplied as to harm pastoral effectiveness. Such would be the case, for example, if because there were too many Masses, only small groups of the faithful would attend each one in churches that can hold many people; or if, also because of the number of Masses, the priests were to be so overwhelmed with work that they could fulfill their ministry only with great difficulty.
Masses with Particular Groups
27. So that the unity of the parish community may stand out in the eucharist on Sundays and holydays, Masses for such particular groups as parish societies should, if possible, preferably be held on weekdays. If they cannot be transferred to weekdays, care should be taken to maintain the unity of the parish community by incorporating these particular groups into the parish celebrations.
Sunday and Holyday Masses Anticipated on the Previous Evening
28. Where indult of the Apostolic See permits fulfillment on the preceding Saturday evening of the obligation to participate in the Sunday Mass, pastors should carefully teach the faithful the meaning of this favor and should take steps to prevent its lessening in any way the sense of what Sunday is. This concession is meant to enable the faithful in today's conditions to celebrate more easily the day of the Lord's resurrection.
All concessions and contrary customs notwithstanding, this Mass may be celebrated only on Saturday evening, at hours to be determined by the local Ordinary.
On the Saturday evening, the Mass is to be celebrated as assigned in the calendar for Sunday and the homily and general intercessions are not to be omitted.
All these points apply also to the celebration of Mass that, for the same reason, is anywhere allowed on the evening before a holyday of obligation.
The evening Mass before Pentecost Sunday is the Mass of the Saturday vigil with the Credo. Likewise the evening Mass before Christmas is the Mass of the vigil celebrated in a festal way with white vestments and with the Alleluia and the preface from the Mass of the Nativity. The evening Mass before Easter may not be started before dusk or certainly not before sunset. This Mass is always the Mass of the Easter Vigil, which by reason of its special significance in the liturgical year and in the whole Christian life must be celebrated with the liturgical rites for this holy night according to the rite for the Easter Vigil.
The faithful who begin to celebrated the Sunday or holyday of obligation on the evening of the preceding day may go to holy communion even if they have already done so that morning. Those who "receive communion during the Mass of the Easter Vigil or during the MAss of the Lord's Nativity may receive again at the second Mass of Easter and at one of the Day Masses of Christmas."  Likewise "the faithful receiving communion at the chrism Mass on Holy Thursday may receive again at the evening Mass on the same day," in accordance with the norm of the Instruction Tres abhinc annos, 4 May 1967, no. 14.
Mass Celebrated on Weekdays
29. The faithful should be invited to take part in Mass often on weekdays as well, even daily.
This is especially recommended for those weekdays that should be celebrated with particular attention, above all in Lent and Advent; also on lesser feasts of the Lord and on certain feasts of the Blessed Virgin Mary or of the saints that are held in special honor in the universal or the local Church.
30. It is very fitting that meetings or congresses aimed at fostering the Christian life or the apostolate or at promoting religious studies, as well as spiritual retreats of various kinds, should be planned in such a way that the eucharistic celebration is their high point.