For us on the Old Calendar this Saturday is Memorial Saturday of St. Demetrius:
H/T: Orthodox Church in America (here)
In the spiritual experience of the Russian Church, veneration of the holy Great Martyr Demetrius of Thessalonica is closely linked with the memory of the defense of the nation and Church by the Great Prince of Moscow, Demetrius of the Don (May 19).
St Demetrius of the Don smashed the military might of the Golden Horde at the Battle of Kulikovo Field on September 8, 1380 (the Feast of the Nativity of the Most Holy Theotokos), set between the Rivers Don and Nepryadva. The Battle of Kulikovo, for which the nation calls him Demetrius of the Don, became the first Russian national deed, rallying the spiritual power of the Russian nation around Moscow. The “Zadonschina,” an inspiring historic poem written by the priest Sophronius of Ryazem (1381), is devoted to this event.
Prince Demetrius of the Don was greatly devoted to the holy Great Martyr Demetrius. In 1380, on the eve of the Battle of Kulikovo, he solemnly transferred from Vladimir to Moscow the most holy object in the Dimitriev cathedral of Vladimir: the icon of the Great Martyr Demetrius of Thessalonica, painted on a piece of wood from the saint’s grave. A chapel in honor of the Great Martyr Demetrius was built at Moscow’s Dormition Cathedral.
The St Demetrius Memorial Saturday was established for the churchwide remembrance of the soldiers who fell in the Battle of Kulidovo. This memorial service was held for the first time at the Trinity-St Sergius monastery on October 20, 1380 by St Sergius of Radonezh, in the presence of Great Prince Demetrius of the Don . It is an annual remembrance of the heroes of the Battle of Kulikovo, among whom are the schemamonks Alexander (Peresvet) and Andrew (Oslyab).