In the fifteen years I spent as a Catholic before becoming a priest, I encountered the Roman Canon (Eucharistic Prayer I) at Mass very, very rarely. For whatever reasons that I don't understand, I guess that there are large parts of the praying catholic world that almost never encounter this prayer. Some anecdotal evidence from my own life illustrates what I'm talking about:
When I was a postulant I went to Mass with one of my classmates on the feast of the Epiphany. The priest prayed the Canon. Having coffee at a diner after Mass, my brother postulant expressed his annoyance and confusion at the priest who had "made up his own prayer." My classmate, a cradle catholic, had never heard Eucharistic Prayer I before!
When I was in theological studies we had to take a course to learn how to offer the Mass. When one of my fellows took his turn practicing the Mass in front of the class, he used Eucharistic Prayer I. The professor, clearly disgusted, scolded him and let us know that the Roman Canon would not be acceptable for the purposes of the course.
So, when I got to be a new priest who wanted to make use of this most Roman Eucharistic Prayer, I faced a problem. Most priests I had known and prayed with would almost never use it, so when should I? I didn't want to make it a case of whim, so I needed a plan.
Here's the plan I have adopted for the use of Eucharistic Prayer I, the Roman Canon. I use Eucharistic Prayer I:
1. On any day for which it provides proper Communicantes and/or Hanc Igitur, excepting nuptial Masses. (Though I would certainly oblige if a bride asked for the Roman Canon.) This amounts to Christmas and Easter and their octaves, Epiphany, Holy Thursday, Ascension, and Pentecost.
2. On the feast of any saint who is named in it, excepting feasts of Our Lady, as she is named in every Eucharistic Prayer.
3. On any feast that has a particularly Roman character, e.g. The Chair of Peter, the dedication of the Lateran, etc.
All together this makes for something like twelve percent of the whole liturgical year.