Traditionally, January 18 was the feast of the Chair of St. Peter at Antioch (now suppressed), which bookended the feast of St. Paul's Conversion on January 25. The too great evangelizers, one in Christ, though given different missions by the Risen Lord, thus rooted the prayer for the unity of Christians.
[BTW, as February 22 was the feast of the Chair of St. Peter in Rome, a folkloric saying in these parts had it that the worst winter storms took place "between the chairs". ]
Some countries or groups of churches choose different starting and closing dates so as to have two Sundays in the octave (eight days) of prayer for unity, so in Ottawa on Sunday there was an evening of prayer for unity at the Ottawa Mennonite Church on Kilborn and next Sunday there will be a similar service at St. Paul's Anglican Church in Kanata.
Here is some information on this year's theme:
Week of Prayer for Christian Unity
At least once a year, many Christians become aware of the great diversity of ways of adoring God. Hearts are touched, and people realize that their neighbours' ways are not so strange.
The event that touches off this special experience is something called the Week of Prayer for Christian Unity. Traditionally celebrated between January 18-25 (in the northern hemisphere) or at Pentecost (in the southern hemisphere), the Week of Prayer enters into congregations and parishes all over the world. Pulpits are exchanged, and special ecumenical worship services are arranged.
Ecumenical partners in a particular region are asked to prepare a basic text on a biblical theme. Then an international group with WCC-sponsored (Protestant and Orthodox) and Roman Catholic participants edits this text and ensures that it is linked with the search for the unity of the church.
The theme for 2011: "One in the apostles’ teaching, fellowship, breaking of bread and prayer" (Acts 2.42-47)
The Week of Prayer for Christian Unity began in 1908, when an Episcopalian priest, the Rev. Paul Wattson, encouraged members of his religious order to pray for Christian unity. In 2008, the Canadian Council of Churches compiled a collection of prayers used throughout the movement in Liturgies for Christian Unity (Novalis).
Since 1968, the Week of Prayer for Christian Unity has been jointly coordinated by the Vatican Pontifical Council for Promoting Christian Unity and the World Council of Churches, through the WCC's Commission on Faith and Order, which also accompanies the entire production process of the text. The final material is sent to member churches and Roman Catholic dioceses, and they are invited to translate the text and contextualize it for their own use. The organizers encourage churches to adapt the materials to their local context and to use the prayers at any time of year.
As we pray for the common life and witness of the Christian churches, in response to Jesus' prayer that we 'all be one' we are reminded of what unites us: God's calling of us all, in Christ, by the power of the Holy Spirit, to serve God's mission in the world around us.