From Pope Benedict's Ash Wednesday catechesis:

it is above all in the liturgy, in participation in the holy mysteries, where we are led to undertake this journey with the Lord; it is putting ourselves in Jesus' school, reflecting on the events that brought us salvation, but not as a simple commemoration, a memory of past events. In the liturgical actions, where Christ makes himself present through the power of the Holy Spirit, those salvific events become actual. There is a key word to which recourse is often taken in the liturgy to indicate this: the word "today"; and it must be understood in its original, not metaphorical sense. Today God reveals his law and lets us choose today between good and evil, between life and death (cf. Deuteronomy 30:19); today "the Kingdom of God is at hand; repent, and believe in the Gospel" (Mark 1:15); today Christ died on Calvary and has resurrected from the dead; he has ascended to heaven and is seated at the right hand of the Father; today we are given the Holy Spirit; today is the favorable time. To participate in the liturgy means, therefore, to submerge one's life in the mystery of Christ, in his permanent presence, to undertake a journey in which we enter into his death and resurrection to have life.