... The music of the organ only, or the harmonium, is allowed on the III Sunday of Advent and the IV Sunday of Lent ...
And Advent requires only musical 'moderation'. @JonathanKK is correct for the EF.305. ... During Lent it is forbidden for the altar to be decorated with flowers. Exceptions, however, are Laetare Sunday (Fourth Sunday of Lent), Solemnities, and Feasts.
313. The organ and other lawfully approved musical instruments ... In Lent the playing of the organ and musical instruments is allowed only in order to support the singing. Exceptions, however, are Laetare Sunday (Fourth Sunday of Lent), Solemnities, and Feasts.
The Society of St Gregory (SSG) has published a Processional (updated 2012) "with the approval of the Dept of Christian Life and Worship of CBCEW" This gives translations by ICEL or SSG of RM, GR and GS antiphons (and psalms), no tunes. No church or cathedral that I attend appears to use it except Westminster Cathedral (but perhaps I don't get about enough). It clearly has not stopped four hymn sandwich yet, (and that includes Metropolitan Cathedrals).... In the dioceses of England Wales the Entrance Chant may be chosen from among the following: the antiphon with its Psalm from the Graduale Romanum or the Graduale Simplex, or another chant that is suited to the sacred action, the day, or the time of year, and whose text has been approved by the Conference of Bishops of England and Wales.
If there is no singing at the Entrance, the antiphon given in the Missal is recited ...
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