This is an old preface tinkered with in the 1970s. It is provided, in several Sacramentaries, either for the last Sunday after Epiphany or for Sepuagesima itself. So I suppose it is ... sort of ... seasonal.
Praefatio III de Dominicis per Annum.
VD ... omnipotens aeterne Deus: Ad cuius immensam gloriam pertinere cognoscimus ut mortalibus tua Deitate succurreres; sed et nobis provideres de ipsa mortalitate nostra remedium, et perditos quosque unde perierant, inde salvares, per Xtm Dnm nostrum.
I first started thinking about this ... you know how it is ... because I couldn't think of the answer to a rather obvious question which a III Former could probably suss: why are the subjunctive verbs in Historic Sequence (i.e. Imperfect Subjunctives)? I still haven't shifted this log-jam in my mind ...
In despair, I ended up, as one does, looking at the Verona Sacramentary, which I suspect has the earliest known version of this preface (beginning of October). Basic differences are these: for the "pertinere cognoscimus" VS simply had "pertinet"; and the subjunctive verbs were in the Perfect Subjunctive: "succurreris ... provideris ... salvaris". These perfect subjunctives seem already to have mutated into imperfects in the Sacramentarium Bergomense and the 'Gregorian' Missal. [Salvaris is by a common syncope for salvaveris. One source, incidentally, has its knickers in a real twist: it reads succurras.]
Are we to interpet the Verona Sacramentary version as "It pertains to your ginormous glory that you have succoured ... have provided ... have saved ...?" This seems to me to make better sense and grammar than the (I suspect) subsequent alterations. It is, indeed, roughly how current ICEL actually translates the formula.