In this context, the word "mystery" means a transcendent purpose of God. It exceeds human understanding, but we have some knowledge and experience of it. Paschal means pertaining to Easter (the Pascha) and to its antecedent the Hebrew Passover. The Passover has the promise of redemption and the gift of freedom at the Red Sea. In the NT, the Paschal concept includes Jesus' death and resurrection, the ascension and gift of the Holy Spirit, baptism, the calling of a new people from every nation and language, and participation in the mystery through eating and drinking with our Risen Lord. The creation enhances our sense of the Paschal Mystery with the gift of light (including the Paschal full moon), with the beauty and wonder of nature, and our own creation in God's image (and in the northern hemisphere, the arrival of Spring). These meanings are drawn together in the Great Vigil of Easter, the most comprehensive and dramatic liturgy of the church (BCP, pp. 284-295). Baptism and Eucharist, and the other rites of the church, celebrate the Paschal Mystery more briefly in all times and seasons.
Glossary definitions provided courtesy of Church Publishing Incorporated, New York, NY,(All Rights reserved) from "An Episcopal Dictionary of the Church, A User Friendly Reference for Episcopalians," Don S. Armentrout and Robert Boak Slocum, editors.