177. The third commandment is: To sanctify the feasts.
There are holidays the Sundays and some other days ordered by the Church.
In the old law the Sabbath was sanctified.
In the new law, Sunday is sanctified because on that day Jesus Christ rose from the dead and the Holy Spirit descended upon the Apostles.
178. In the third commandment we are commanded to honor God with works of Christian piety on the feast days dedicated to His worship.
Sanctifies the feast he who hears the whole Mass and does not work without necessity.
He who on a holy day misses the Mass without just cause commits mortal sin.
Excuses to hear Mass a moderately serious cause.
The Mass must be heard whole and with devotion.
Whole Mass means from beginning to end.
The precept is fulfilled by hearing at different times part of one Mass and part of another; it is appropriate that the Consecration and the Communion are of the same Mass.
Losing from the beginning of the Mass to before Offertory, or the rest after Communion, is a slight fault.
Consecration and Communion are the two essential parts of the Mass; it is enough to attend these two parts to really hear the Mass, although, to fulfill the precept on holidays, one must attend Mass from the beginning to the end.
You can hear Mass and confess at the same time.
Several Masses can be heard at a time, focusing attention on one. [I suppose that was when there were more people and more devotion and one Mass was celebrated on a side altar and another could be celebrated on another altar]
In addition to hearing Mass, it is advisable to occupy the day with good things; attend the sermon, vespers, read some good book, etc.
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Do not work
179. The third commandment forbids corporal labor on Sundays and feast days.
The third commandment forbids working on feast days.
The prohibited jobs are subservience and forensics.
Subservient jobs are those of craftsmen and workers.
Forensic works are legal cases and criminal proceedings, public contracts, fairs, markets, etc.
It is allowed on holidays to read, write, teach, travel [the O.T. says that we must not even prepare our trips on Saturday], draw, hunt (without much noise), fish, etc.
[But if you hunt all week, on Sunday you have to rest, the same if you study from Monday to Saturday, our body and mind need that rest not be exhausted]
All work necessary or convenient for domestic life, such as preparing food, sweeping, cleaning, etc., is also permitted.
[Again, if the poor woman is cleaning all week, she must not clean on Sundays for her own sake, she must do only what is strictly necessary, or better: to let the house be a bit messy.]
Custom allows barbershop work, fairs and markets in some countries. [Custom must not be above divine laws]
180. The one who works or makes work without necessity on Sundays or days of precept, sins mortally if the work lasts much more than two hours, and venially if it lasts less time and does not cause scandal.
[Washing the car on the sidewalk or with the garage open, cutting the grass in the sidewalk or with an electric mower that makes noise in a feast of precept is to make scandal.]
Working more than two hours without just cause on a feast day is a mortal sin.
If it does not exceed two hours is venial sin, usually.
When the work is light, it is not a mortal sin, if it does not reach three hours.
The just causes for which it is allowed to work on a public holiday are charity towards one's neighbour [e.g. visit the sick and help them in something if he cannot be visited and helped during the week], piety towards God and need.
When for some just cause work must be done, it is advisable, though not obligatory, to ask permission from the parish priest.
In Feast days, idleness and dangerous diversions should be avoided.
One can take some honest and moderate rest after having fulfilled religious duties.
The sanctification of the feast is useful not only to the soul, but also to the body; for rest contributes much to the preservation of health.
Not sanctifying the feasts is one of the sins that most attract the righteous punishments of God, even in this life.