In the Christian liturgical calendar, there are several different feasts known as Feasts of the Cross, all of which commemorate the cross used in the crucifixion of Jesus.
The original name of this feast was the Feast of the Exaltation of the Cross, by which name it is still know by the Orthodox Church. Since 1970, it has been officially called the Triumph of the Cross by the Roman Catholic Church. It is also popularly known among Roman Catholics and Anglicans as Holy Cross Day.
The feast commemorates the finding of the True Cross in 326 during a pilgrimage of Christians to Jerusalem. Among those in attendance at this pilgrimage was St. Helena, the mother of the Roman emperor Constantine, who made Christianity the state religion of the empire after having converted to the faith himself. The Church of the Holy Sepulchre was then built at the site of the discovery, by order of Helena and Constantine. The church was dedicated nine years later, with the cross placed inside it. In 614, the cross was carried away from the church by the Persians, and remained missing until it was recaptured by the Byzantine Emperor Heraclius in 628. The cross was returned to the church the following year after initially having been taken to Constantinople by Heraclius.
The date used for the feast marks the dedication of the Church of the Holy Sepulchre in 335. This was a two day festival: althought the actual consecration of the church was on September 13, the cross itself was brought outside the church on September 14 so that the clergy and faithful could pray before it.
In Roman Catholic liturgical observance, the Triumph of the Cross belongs to the same class of holy days as Good Friday and the Feast of the Sacred Heart, in that the Crucifixion is commemorated on all three days. Red vestments are worn at church services conducted on this day, and, in the Roman Catholic calendar, if the day falls on a Sunday, its Mass is used instead of that for the Twenty-Fourth Sunday in Ordinary Time which would otherwise fall during that week.
Prior to 1970 in the Roman Catholic Church and 1976 in the Anglican churches, the Wednesday, Friday and Saturday of the calendar week after the one in which September 14 falls were designated as one of each year's four sets of Ember days.
In Orthodox practice, the Exaltation of the Cross commemorates both the finding of the Cross in 326 and its recovery from the Persians in 628. One of the high points of the celebration is the singing of the festal troparion "Before your Cross, we bow down in worship", as all the members of the congregation prostrate themselves on the ground.
In the Gallician usage, beginning about the seventh century, the Feast of the Cross was celebrated on May 3. According to the Catholic Encyclopedia, when the Gallician and Roman practices were combined, the September date was used to commemorate the rescue from the Persians and the May date was kept as the Finding of the Holy Cross or Invention of the True Cross to commemorate the finding. (Note: the term "Invention" is from the Latin invenire, to find, and should not be understood in the modern sense of creating something new.)
This feast was removed from the General Roman Calendar effective in 1970 by the Second Vatican Council as part of its policy of either moving or abolishing feasts that would typically occur during Advent or Lent, or in the time between Christmas and the Epiphany or in the time between Easter and Pentecost (May 3 always falls during the latter). However, it is still commemorated in some Catholic parishes, and the churches of the Anglican Confession (such as the Church of England and the Episcopal Church in the United States) still recognize it as an optional observance.
The Orthodox commemorate the Feast of the Procession of the Venerable Wood of the Cross on August 1st, the day on which the relics of the True Cross would be carried through the streets of Constantinople to bless the city [ (http://www.oca.org/pages/orth_chri/Feasts-and-Saints/August/Aug-01.html)].
In addition to celebrations on fixed days, there are certain days of the variable cycle when the Cross in celebrated. The Roman Catholic Church has a formal Adoration of the Cross during the services for Good Friday, while the Orthodox celebrate an additional Veneration of the Cross on the third Sunday of Great Lent.