The tradition of visiting seven churches on Holy Thursday is an ancient practice, probably originating in Rome, where early pilgrims visited the seven major basilicas as penance. They are Saint John Lateran, Saint Peter, Saint Mary Major, Saint Paul-outside-the-Walls, Saint Lawrence-outside-the-Walls, Saint Sebastian-outside-the-Walls, and Holy Cross-in-Jerusalem.
Pope Boniface VIII revived the pilgrimage tradition in 1300 with the establishment of the Jubilee Year, and Rome became a center for pilgrimages. The seven churches developed as particular sites for visits. The seven areas may also be related to the historical seven deacons of Rome, whose responsibility was to minister to the poor in their region of the city. The first seven deacons in the Church were the seven assistants ordained by the Apostles to minister to the poor.
Click here for more information from the Diocese of Pittsburgh, including a mediatation booklet and list of churches open for prayer on Holy Thursday night.