In short, it's up to the celebrant. Either choice is valid. It's also possible to keep the readings of the day, but use the propers of the feast, which actually only ranks as an optional memorial. Also, even if Fatima is celebrated liturgically, there are no assigned propers, but you would choose from the common of BVM.
I'd recommend neither, though they would be permitted (but not required) for an Easter weekday, but instead recommend choosing from the readings provided in the Order of Confirmation. I don't think the centenary of the first apparition is sufficiently supervening for that particular choice.
Considering the the Confirmation Mass was made up a few years ago... it is not overwhelmingly important. I would go for and suggest the Mass of the BVM pointing out the Rome thinks this is a good idea, N.B. It is incredibly rare for such a decree to be issued.
The decree in question concerns celebrations in the Extraordinary Form, whereby St. Robert Bellarmine's feast day is usually celebrated that day. Because of the centenary of the Fatima apparitions, permission has been given to use the Votive Mass of the Immaculate Heart of the BVM on this day whereas under the usual circumstances this would not be possible. In the Ordinary Form, Our Lady of Fatima is already on the calendar as an optional memorial. Since it is an optional memorial, I think the Ritual Mass for Confirmation, with its propers, readings, etc., should be used. Perhaps singing the "Regina caeli" at the conclusion would be fitting.
In addition to the Regina Caeli, I'd also add my usual bleat about the Magnificat - receiving a gift of the Holy Spirit is part of the original context, albeit the Annunciation rather than Pentecost. But the ritual Mass would be better served if the readings were taken from the Order of Confirmation.
When I read the original post, I didn't realize it was a Ritual Mass for confirmation, but thought he was asking "confirmation" on which prayers/readings to use. So really, in the ordinary form there would be three possibilities depending on the preference of the bishop. This is why planning "proper" music is so difficult in the ordinary form since there are so many options.
Well, I assumed it was a Ritual Mass for Confirmation because Saturdays in the latter half of Eastertide are a very common date for such Ritual Masses (while First Communion Masses are often celebrated on the third or fourth Sundays of Easter).
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