The Cenacle room on Mt Zion in Jerusalem is where two major events in the early Christian Church are commemorated: The Last Supper and the coming of the Holy Spirit on the apostles.
• The Last Supper was the meal Jesus shared with his apostles the night before he died. During this meal he instituted the Eucharist.
• The coming of the Holy Spirit, at Pentecost, is recognised as marking the birth of the Christian Church.
The Cenacle is on the upper floor of a two-storey building near the Church of the Dormition, south of the Zion Gate in the walls of Jerusalem’s Old City.
Where Peter was left knocking
According to early Christian tradition, the “upper room” was in the home of Mary the mother of John Mark. He was the author of the Gospel of Mark (and presumably also the young man who fled naked, leaving behind his linen garment, to escape the authorities when Jesus was arrested in the garden at Gethsemane, an event he recorded in Mark 14:51).
This house was a meeting place for the followers of Jesus inside the city walls of Jerusalem.
It was also the house to which Peter went after an angel of the Lord released him from prison. Acts 12:12-16 says a maid named Rhoda was so overjoyed at recognising his voice that she left him knocking at the outer gate while she went to tell the gathered disciples.
Above Taken from See the Holy Land