The great Patron of Cornubia seu potius Dumnonia, Cornwall, Britannia Maior: S Petroc the Abbot: is listed on the Ordinariate Calendar for May 23. I find this is a trifle odd. In Catholic Cornwall and Devon, he was listed on 4 June (at Tavistock Abbey, Launceston, and St Michael's Mount). The Exeter Martyrology has the same date; and so does the Sarum Breviary of 1531. Not to mention a couple of North French liturgical books.
And, following the erudite Canon Doble, the Anglicans keep him on that day.
There are three English Calendars which do observe the Patron of Cornwall on May 23; one is from Croyland and another is the Bosworth psalter. But ...
Why should the Ordinariate want to go off on this limb?
The Roman Martyrology gives S Petroc his pretty universal date of June 4. I don't see any pressing reason to ignore this ruling.
Tell you what I'll do: In the Ordinariate, S Petroc is optional on May 23, so I'll lawfully miss him out on that day. But, in accordance with overwhelming precedent and the 'new' CDF rules and permissions of 2020, I'll observe him on June 4.
He was also observed, in some places, on Holy Cross Day; with his 'Translatio' [to Bodmin; to get his relics away from coastal raiders] on October 1. His Collect: Gloriosi famuli tui Petroci, Domine, precibus pacem tuam nostris concede temporibus: et Ecclesiae tuae hanc largire misericordiam; ut nullus tuis privetur beneficiis qui huius Confessoris expetit opitulari suffragiis."