Good question, no easy answer! For Mass, since I habitually go to the monastery, I am more or less forced into their calendar. For instance if I go to Mass at the monastery on the Sunday that is Corpus Christi in my parish, the propers, ordinary and readings will be of the corresponding Sunday in Ordinary Time. The same thing happens at Epiphany, as the monastery celebrates it on January 6th but the Conference of Bishops transfers it to the Sunday. If I choose to go to my parish that Sunday but have also been to Corpus Christi at the monastery on Thursday, I get to celebrate it twice.
So the calendar for the Mass ends up being the calendar that you attend. And there’s no obligation to attend at either the monastery or the parish. There is just the Sunday obligation to respect.
Where it gets tricky is the Liturgy of the Hours. If I am praying the Roman Office (which I’m doing right now as I’m busier in the summer), then I typically use the diocesan calendar but will do Corpus Christi on the Thursday to be consistent. The issues arise with the calendar of saints. Some monastic saints are not even options memorials in the Roman calendar, and there is no common of monks and nuns in the LOTH, but there is in the Monastic LOTH. So what I do is celebrate the monastic saint as if he or she was an optional memorial. The rubrics allow one to celebrate any saint in the martyrology as an optional memorial if the day isn’t otherwise interdicted. To do so, I use the gospel canticle antiphon and the collect of the saint at Lauds and Vespers, and at the Office of Readings, the hagiographic reading and collect of the saint. That’s also a licit way to do it. For a memorial without propers, you can limit it to the collect.
Another obstacle is the feast of Saint Benedict on July 11th. On the monastic calendar, it’s a solemnity. On the Roman, a mandatory memorial for most, but a feast in Europe. I usually do it as a feast for practical reasons (the limitations of the antiphonary I use). There’s also the feast of his passing on March 21st. I use the July 11th texts and music except for the collect, which I lift from the monastic books. Unless I am doing the monastic office at that time,
There’s always a bit of a ying and yang pull between the two offices and calendars. Adding to the complexity, I work at the abbey every Wednesday in the library and usually am there for the daytime hours, Mass, and Vespers. So if I pray the Roman, it throws me out of synch, so I usually also pray the Roman in silence on my noon break and just before Vespers.
