From the World Council of Churches Website:
At least once a year, Christians are reminded of Jesus’ prayer for his disciples that “they may be one so that the world may believe” (see John 17.21). Hearts are touched and Christians come together to pray for their unity. Congregations and parishes all over the world exchange preachers or arrange special ecumenical celebrations and prayer services. The event that touches off this special experience is the Week of Prayer for Christian Unity.
WHEN: Traditionally the week of prayer is celebrated between 18-25 January, between the feasts of St Peter and St Paul. In the southern hemisphere, where January is a vacation time, churches often find other days to celebrate it, for example around Pentecost, which is also a symbolic date for unity.
the Reverend David Taylor, co-chair Ecumenical/Multi-Faith Unit