It was one of the agreed initiatives that came out of that founding conference.
It has had major take-up in many countries in Europe for a very long time. In many of the countries of the former communist world in Europe it was the only Evangelical Alliance thing that stayed alive through the communist era…..so, even when people had heard of nothing else to do with Evangelical Alliance, they had often heard of the Week of Prayer. It pre-dates the ecumenical ‘Week of Prayer for Christian Unity’ by about a century.
Historically it has been in the first full week of January each year.