Observationis huius annua celebritate laetantes,
ut, paschalibus sacramentis inhaerentes,
plenis eorum effectibus gaudeamus.
Today we have a nice parallel of the forms laetantes and inhaerentes and also of paschalibus sacramentis and plenis effectibus.
This was in the Gelasianum Vetus on Saturday of Quinquagesima: Obseruationis huius annua caelebritate laetantes, quaesumus, domine, ut paschalibus actionibus inherentibus plenis eius effectibus gaudeamus. Notice the word is actio and not sacramentum. In the Veronese a version is found in the month of July: Obseruationis annuae celebritate gratulantes… ut eorum, quorum actionibus inheremus, plenis effectibus gaudeamus. Again we have actio. There was no version of this prayer in the pre-Conciliar Roman Missal.
An observatio, according to Blaise is "observance", particularly of Lent. In the L&S we go more deeply into the word and find "observance, attention, respect, regard, reverence shown to another" also, "an observance of religious duties, divine worship, religion". Actio in Blaise is "the celebration of the mysteries, the holy sacrifice". This is why the Roman Canon with the Preface was called the canon actionis. However, actio was replaced by the redactors of the Novus Ordo with sacramentum in the plural. The paring sacramenta paschalia is very Leonine, appearing quite a few times in that great Pope’s sermons. Antoine Dumas, who reworked Blaise’s original, suggests that this combination would be "paschal mysteries", which seems entirely appropriate.
Rejoicing in the in the annual celebration of this religious observance,
we pray, O Lord,
that engaging deeply in the paschal mysteries,
we may rejoice in their full effects.
Here "effects" limps a bit for effectus which, in this context, points to the realization of both the Lenten observance and the paschal mysteries in both the sense of the Lent/Easter dying and rising of the Lord and also the sacred action of Holy Mass. Effectus must be the desired effective outcome of the the preceding. I think we get at this idea adequately with "effects" in English, though we might really need a circumlocution like, "effective outcome", or somewhat more philosophically "efficient realization".
The vocabulary and theme of the prayer seems to sum up something of what has been going on in the previous Collects of the week. Perhaps the pattern we are seeing is that the Saturday prayer, being the last of the week, ties up the themes of the rest of the week, before moving to the new ideas to be brought forth starting with Sunday, tomorrow.