What this source establishes
⭐ FR GRÉGOIRE'S OWN PRIVATE NOTEBOOK, quoted in the parish English history p.43 — the only account of the opening that exists, and it is the building priest's own. It gives the two days, the arrival with Fr Verdier and Fr Boyer, the reception at the chapel of St Aloysius, Frs Poughet, Guchen, Fernandez and Victor, the five o'clock benediction, Br Bergenthal present at the blessing of the church he had drawn, the bishop's silver jubilee kept here, and the Pontifical Mass of 29 June at which the church was DECLARED DEDICATED TO THE HOLY FAMILY — the dating of the titular this church still carries. Transmitted through the parish's own book, so parish-side, but a contemporary first-person document behind it.
This is as much of the book as this site prints. It is named in full in the bibliography below.