At that time, the Bulgarian nation was still being formed from the amalgamation of pagan Bulgarians, who mostly controlled the government, a Slav majority, and what had remained of the ancient Thracians. In 863, Prince Boris I adopted Eastern-rite Christianity as a means to unite the constituent groups who prayed to different pagan gods. By the 880s, he had already realized that adopting Christianity from Byzantium had paved the way for unwanted Greek political influence in Bulgaria.
When Prince Boris I heard that Cyril and Methodius’s disciples had reached his realm, he was quick to summon them to his capital. He realized that the Glagolitic alphabet and the use of the Slavonic language in church would be an effective tool against Byzantium. In an ironic twist of fate, Prince Boris used Byzantium’s political tool against Byzantium itself.
The disciples arrived in Bulgaria in such bad health that only two of them survived the ordeal, but the work of Konstantin in Preslav, and of Kliment in Ohrid, now in North Macedonia, had a dramatic effect. They versed dozens of young boys in the new script and laid the foundations of Bulgaria’s own, Christian culture. Used both in church and by the administration, the Slavonic language bonded the nation.
In the first half of the 10th century, Bulgarian scribes replaced the Glagolitic with a simpler alphabet, based on the Greek script. The Cyrillic kept some Glagolitic letters, like the Hebrew borrowings shin, or ש, that had become the ш, and tsade, or צ which was now the ц.
Soon Bulgarians started spreading Christianity to other Slavs, like the Russians, using the alphabet to help their mission. Today, the script is used in Bulgaria, Serbia (concurrently with the Latin script), North Macedonia, Ukraine, and Russia. Even non-Slavic-speaking Romania used it until the early 19th century.
The Glagolitic alphabet survived in Croatia for centuries and was used for religious purposes as late as the 20th century.
The achievement of Cyril and Methodius was recognized even by their contemporaries. The Catholic Church celebrates its feast day on 14 February, the day when Cyril died. In 1980, they were declared co-patron saints of Europe. In East Orthodoxy, their feast day is on 11 May. Their disciples also became saints in Bulgaria.
In the following centuries, the Cyrillic alphabet became one of the pillars of Bulgarian identity. It proved crucial during the 500 years of Ottoman domination when Bulgarians lacked official institutions of their own and were again threatened with cultural assimilation by the Greeks. Amazingly, in the 18th and 19th century, when many Bulgarians were again being seduced by the more refined Greek culture, the Slavonic alphabet was what saved the nation, 1,000 years after its invention. In the mid-19th century, Bulgarians started to celebrate Ss Cyril and Methodius Day very publicly and with great aplomb. It was one of the first Bulgarian-only days celebrated by Bulgarians, a demonstration of identity and a rallying cry for Bulgarian nationalism.
When the Bulgarian state was restored, in 1878, the Day of Ss Cyril and Methodius increased in popularity. It became also a day of Slavonic and Bulgarian literacy and culture and was marked by open-air liturgies followed by mass rallies of school pupils, students, and everyone working in education, science, and culture.
After the 1944 Communist coup, the day remained on the calendar but changed to fit the new state ideology. The rallies remained, but the liturgies were removed. The “saints” were dropped from the name of the holiday, and it became simply the Day of Cyril and Methodius.
Alphabet Day continues to be a major celebration of youth and education in Bulgaria, and Bulgarian schools have their proms around that time. Although the religious holiday is on 11 May, the official institutions and the overwhelming majority of Bulgarians celebrate it in the New Style, on 24 May. In North Macedonia and Russia Ss Cyril and Methodius are also venerated on this date while in Czechia and Slovakia, the feast is on 5 July.