Concerning time, St. Augustine says he knows that it is, but not what it is. The continued debates in our day recount this same difficulty. Yet, though we don't know precisely what time is, this does not mean that the Church cannot redeem it for God’s glory. To do this, the Church dedicates days, weeks, months, and years. We have a Liturgical Calendar with particular celebrations for our Lord, the Saints, and Seasons. The classical division of the day into "hours" is sanctified by each “hour” of the Divine Office. And, in the Holy Sacrifice of the Mass, temporality is both penetrated and permeated by eternity.
In this consecration of time to the glory of God, the Church uses two special types of "days" to express the untold joy that ought to emanate from the Christian life. The Triduum is a uniting of three temporal days into one liturgical day. It is a statement that the mystery being celebrated is so tremendous that time cannot contain it. Church history has seen multiple uses of the Triduum, but in our current Liturgical Calendar it is reserved formally for the last three days of Holy Week (Maundy Thursday, Good Friday, Holy Saturday). It is as if the event of the Crucifixion of Our Lord was so traumatic to the natural order that it fused the day before and the day after together into one for all time. The Church recognizes this mystical reality in the liturgical formulas of the Easter Triduum. Liturgically, the Mass of the Lord's Supper, the Liturgy of the Presanctified, and the Easter Vigil are all considered one action. This attests to what is said in the blessing of the Easter Candle at the Vigil when the priest traces the year on it, saying, "To Christ belongs all time and all the ages." For the Master of time is Christ.
The Octave is less a statement about time and more about joy. The Easter Octave begins with Easter Sunday and ends on Quasimodo Sunday (a.k.a. the Second Sunday in Easter, among other names). Unlike the Triduum, it is not a fusing of multiple days into one, but instead is the repetition of the feast for eight days. If the Triduum is an expression of divine action, then the Octave is a statement about our response to that action. We commemorate the day of the Resurrection, the Eighth Day of Creation--where, by the Pasch of Christ and Baptism, the soul is recreated and elevated higher than the angelic powers by God’s grace--for eight days. But the Octave is not only a human response. It is also a Divine promise. It is a promise to abide in time unto the consummation of all things at the end of time. It is a symbol of Christ's promise to be with us always. And so, the light remains lit in our Sanctuaries and in our hearts for the whole of the Octave.