One theory is that his veneration was transferred from Sirmium when Thessaloniki replaced it as the main military base in the area in 441/442 AD. His very large church in Thessaloniki, the Hagios Demetrios, dates from the mid-5th century. Thessaloniki remained a centre of his veneration, and he is the patron saint of the city.
After the growth of his veneration as saint, the city of Thessaloniki suffered repeated attacks and sieges from the Slavic peoples who moved into the Balkans, and Demetrius was credited with many miraculous interventions to defend the city. Hence later traditions about Demetrius regard him as a soldier in the Roman army, and he came to be regarded as an important military martyr. Unsurprisingly, he was extremely popular in the Middle Ages, and along with Saint George, was the patron of the Crusades.
Some scholars believe that for four centuries after his death, St. Demetrius had no physical relics, and in their place an unusual empty shrine called the "ciborium" was built inside Hagios Demetrios. What are still purported to be his remains subsequently appeared in Thessaloniki, but the local archbishop John, who compiled the first book of the Miracles ca. 610, was publicly dismissive of their authenticity. These are now also kept in Hagios Demetrios. According to believers, these relics were ascertained to be genuine after they started emitting a liquid and strong scented myrrh. This gave the saint the epithet "Myrovlētēs" (Greek: Μυροβλήτης, the Myrrh-streamer).
In the Russian Orthodox Church, the Saturday before the Feast of St. Demetrius is a memorial day commemorating the soldiers who fell in the Battle of Kulikovo (1380), under the leadership of St. Demetrius of the Don. This day is known as Demetrius Saturday.
The Bulgarian Orthodox Church and the Bulgarian people revere St. Demetrius on 26 October as Димитровден (Dimitrovden). SOURCE