Rorate suggests there are speculations about the date of the regularisation of the Society of S Pius X. Well; we've been through these agonies before; these games of Grandmother's Steps. But I can't help thinking that March 25, Lady Day, the Feast of the Annunciation, would be eminently suitable.
Why? It was when the Incarnation ... if we dare to use any human verb ... happened. When a minute speck in a Girl's womb was the Creator. The moment of repose which lies at the heart of all the times and of all the universes. Where all History suddenly miraculously focuses all Divine meaning. The instant when His Body the Church began to exist on Earth. The beginning of the last Age which will be concluded by the End.
And there is, of course, another, more particular, reason for the appropriateness of that Day.
Requiem aeternam dona ei Domine, et lux perpetua luceat ei. Pro nobis omnibus deprecetur.