The Circassian Day of Mourning (Adyghe: - ) or the Day of Mouning for the Victims of the Circassian Genocide (often censored in Russian media as Day of Remembrance for the Victims of the Caucasus War) is mourned every year on 21 May in remembrance of the victims of the Russo-Circassian War and the subsequent Circassian genocide by members of the Circassian diaspora. The choice of the date is due to the fact that on 21 May 1864, General Pavel Grabbe held a military parade in the what is now Krasnaya Polyana in honor of the victory in the Battle of Qbaada.

The Russo-Circassian War (Adyghe: Урыс-адыгэ зауэ, romanized: Wurıs-adığə zawə; Russian: Русско-черкесская война; 1763–1864; also known as the Russian Invasion of Circassia) was the invasion of Circassia by Russia, starting in 1763 with the Russian Empire assuming authority in Circassia, followed by the Circassian refusal, and ending 101 years later with the last army of Circassia defeated on 21 May 1864 (O.S), making it exhausting and casualty-heavy for both sides. It was the longest war Russia has ever waged. During and after the war, the Russian Empire employed a genocidal strategy of systematically massacring civilians which resulted in the Circassian genocide where up to 1,500,000 Circassians (85-97% of the total population) were either killed or expelled to the Ottoman Empire (especially to modern-day Turkey; see Circassians in Turkey), creating the Circassian diaspora.During the war, the Russian Empire did not recognize Circassia as an independent region, and as a result, it considered Circassia Russian land which was under rebel occupation, despite the fact that the region was not under Russian control. Russian generals did not refer to the Circassians by their ethnic name, instead, they called the Circassians "mountaineers", “bandits", and "mountain scum".While the war was initially an isolated conflict, Russian expansion through the entire region soon drew a number of other nations in the Caucasus into the conflict. As such, the war is often considered the western half of the Caucasus War.

The war has been subjected to historical revisionism and it has also garnered controversy due to the fact that later Russian sources mostly ignored or belittled the conflict, and Russian state media and officials have gone as far as to claim that the conflict "never happened” and they have also claimed that Circassia "voluntarily joined Russia in the 16th century".