The Hebdomon (Ἕβδομον, “seventh”) was a coastal suburb of Constantinople located on the Via Egnatia seven miles west of the Milion. As the site of a military camp and an imperial palace, many emperors were proclaimed or began their triumphal processions here. Now known as Bakirköy, the surviving monuments of the district include a large open-air cistern and the remains of a circular mausoleum.

The Hebdomon first played a role in imperial ceremony during the reign of Valens (364-378) who was proclaimed emperor here. Others were also proclaimed emperor at Hebdomon, including Arcadius in 383, Theodosius II in 402, Marcian in 450, Leo I in 457, Zeno in 474, Basiliscus in 475, Maurice in 582, Phocas in 602, Leo III in 717, Leo V in 813, and Nikephoros Phocas in 963. An army camp and a tribunal were located on a plain at the Hebdomon that was known as the Kampos, making it Constantinople’s version of the Campus Martius in Rome. Triumphal processions began at the Hebdomon and continued along the Mese towards the Hippodrome and the Great Palace of Constantinople. The monumental column of Theodosius II at the Hebdomon can be seen as the first of a series of imperial columns and monuments of this triumphal route.

The Hebdomon was also an important imperial residence in Late Antiquity. Literary sources mention the Iucundianae (Ioukoundianai) Palace, where the members of Theodosian dynasty frequently resided, and the Magnaura Palace, which seems to have been an audience hall for the Senate. It had also at least one harbor and its own water supply that included a large open-air cistern in the hills to the north. In 391, the Church of St. John Prodomos was built by Theodosius I to house the relic of the head of St. John the Baptist, while the Church of the Prophet Samuel was built during the reign of Theodosius II to house the prophet’s relics. The Hebdomon continued to be a popular imperial residence during the reign of Justinian, who rebuilt both the Iucundianae Palace and the Church of St. John Prodomos. Other church located here included St. Theodote, Ss. Menas and Menaius, and the Holy Innnocents.

The Hebdomon was probably devastated during the Arab sieges of Constantinople, when their fleets anchored there. It was probably again devastated when the Bulgar Khan Krum led his forces to the city walls in 813. Basil I, who began an imperial triumph at the Hebdomon, restored or rebuilt the churches of St. John the Evangelist and St. John Promodos, which had fallen into ruin. Basil II, who also started his triumph celebrating his victory of the Bulgars at the Hebdomon, would later be buried at the Church of St. John the Evangelist, instead of the Church of the Holy Apostles as was customary since the reign of Constantine I. The decision to be buried near here reveals his self-identification as a military figure, as the Hebdomon continued to hold this connotation. While his sarcophagus now longer survives, its inscription has survived in four Late Byzantine manuscripts.

Archaeological remains the modern district of Bakirköy include a large open-air cistern (Fildamı Cistern) as well as remains of an inscribed statue base of Theodosius II and a circular mausoleum. The cistern, which perhaps dates to the 6th or 7th century, is around 127 x 76 meters, with a maximum capacity of roughly 125,000 cubic meters. The Kampos of the Hebdomon is now the location of the Veliefendi Hippodrome. The remains of a church and palace (probably the Church of St. John Prodomos and possible the Iucundianae Palace) that were recorded around a century ago no longer exist.  It is possible that the relics of St. John the Baptist (known by Muslims as Yahya the Prophet) at Topkapı Palace came from this church. Several archaeological finds from this region are now on display at the Istanbul Archaeological Museums.