However, unless the day-before is itself a Solemnity (or another obligatory day), there’s nothing prohibiting an individual person in private (ie, not a cleric or religious person and not a parish) from treating a saint’s day as if it were a solemnity as a matter of personal devotion.
For example, there is no Solemnity of Saint Fiacre. If someone’s middle name is Fiacre, he could celebrate the day as a personal solemnity and pray Evening Prayer I for that saint (using probably the “common of saints” for the appropriate category if there is no proper).
Keep in mind, I’m writing about a personal devotion here.
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