The date of March 3rd marks the date of the signing the Treaty of San Stefano in 1878.
This peace treaty ended the war and was signed between Russia and the Ottoman Empire, which proclaimed Bulgaria as an independent state, albeit a vassal state within the Ottoman Empire.
Bulgaria had become part of the Ottoman Empire in 1396. In the late nineteenth century, a growing tide of nationalism had been spreading across Europe, which had been threatening the break up of the western parts of the Ottoman Empire.
This gave hope to Bulgarians who were longing for independence. In April 1876, an uprising against Ottoman rule broke out in the city of Plovdiv. The revolt quickly spread to other parts of Bulgaria, but it was ultimately unsuccessful.
In April 1876, uprisings in other parts of the Ottoman Empire spread to Bulgaria. The suppression of the revolts by the Ottomans was horrific and when the atrocities were made public to the West and Russia, international condemnation was widespread. On April 24th 1877, Russia formally declared war on the Ottoman Empire. This was followed by a series of military victories by the Russians, which put pressure on the Ottomans to give in to Bulgarian demands for autonomy. On March 3rd 1878, the Treaty of San Stefano was signed, which recognized Bulgarian autonomy within the Ottoman Empire.
The treaty sparked outrage in other European countries, who feared that Russia was becoming too powerful. They pressured the Ottomans to cancel the treaty and instead sign the Treaty of Berlin in July 1878. This treaty resulted in a smaller autonomous Bulgarian state within the Ottoman Empire. Nevertheless, it was a significant step towards Bulgaria’s eventual independence.