Today's History Lesson

Maria Trapp tells us, "We cannot be grateful enough that the Holy Father, Pope Pius XII, has given us back the ancient Easter night! Even as children, we felt that something was not quite as it should be when the Church broke out early in the morning of Holy Saturday in the three-fold Alleluia, while the Gospel told us that Our Lord was resting the grave to rise on Easter Sunday morning. Now the word of the Holy Father has put things straight, and Holy Saturday has regained its ancient character." [source]

Here's Wikipedia's explanation
In 1951, on an ad experimentum basis, Pope Pius XII introduced the Easter Vigil, a new celebration of Easter night by means of De Solemni Vigilia Paschali Instauranda. This has been highlighted as his most important reform, as the Easter ceremonies are the centre of all Christian faith and life.

In antique times Christians had baptised neophytes during a night-long Vigil preceding Easter Sunday. The celebration concluded after dawn by the offering of Mass. In time this Mass became the Mass of Easter Sunday, which was separated liturgically from the observance of the Easter Vigil, itself being anticipated on the morning of Holy Saturday. This practice of celebrating the Vigil on the morning of Holy Saturday was in place by the twelfth century, after the hour of the liturgical observance had already been moved back bit by bit over preceding centuries.

Pius XII restored the older time for the observance of the ceremonies, but most importantly created a dramatically restructured form of the ceremonies. His re-introduction of the Easter Vigil was generally popular, although it faced a cool reception from prelates like Cardinal Siri of Genoa and Cardinal Spellman of New York. Other Christian denominations adopted the popular Roman Catholic Easter ceremonies in later years, an ecumenical influence of Pius XII. The new Easter Vigil reduces the number of prophecies (Old Testament passages read before the blessing of the font and the Mass) from twelve to four.

The rite for blessing the Paschal Candle was also changed: formerly the deacon would process into the church with a triple-branched candlestick known as the arundo (a symbol of the Blessed Trinity), which would be used to light the Easter Candle. With Bugnini's reform to the Easter Vigil, the Paschal Candle itself is carried in procession and the arundo is suppressed. The Candle is no longer blessed during the singing of the Exsultet, though the liturgical text in question refers to the blessing of the candle. The "renewal of baptismal promises", devised for the new Easter Vigil, introduced into the liturgy of the Mass the principle of vernacularism for the first time. (source)