And by this term I mean the use in a particular congregation of both the Extraordinary and the Ordinary Forms of the roman Rite. I have raised before the problems which this involves. They are, perhaps, at their most acute in the weeks before Ash Wednesday. Elsewhere in the year - for example, in Pentecost Week and on the Sunday next before Advent, these problems can be fudged or elided or ignored. But during the Gesima weeks, the problems get lengthy and structural.
In the EF rite, one irrevocably bids farewell to Alleluia with a twofold use at Evensong on the Saturday before Septuagesima. Thereafter, one replaces the Alleluia before the Gospel with the Tract. But in the OF, Alleluia continues until Shrove Tuesday. There is an obvious difficulty in this as, on weekdays perhaps, on one day the Mass is EF, on the next OF. Here at S Thomas's, we have an additional little local difficulty in as far as, while using the Three Year Lectionary on Sundays, we take the sung propers from the English Gradual (which gives the Roman/Sarum propers rendered into the texts of the Prayer Book Psalter and the Authorised Version).
Then there is the question of liturgical colour. My recollection from last year is that the Oxford Oratory sings Sunday Evensong of the Gesimas, but in green copes.
Life would be simpler if the Congregation for Divine Worship decreed - perhaps optionally - that the Gesima weeks would be Purple and Alleluia-free in the OF as well as in the EF.