In A.D. 863, two brothers named Cyril (Кирилл) and Methodius (Мефодий) were sent by the Byzantine emperor as missionaries to spread the Christian faith to Slavic tribes. Their mission to Moravia lasted only a few decades. They developed an alphabet and translated parts of the Gospels into the local Slavic dialect. After Cyril and Methodius died, their disciples went to South Slavic regions (what is now Serbia and Bulgaria) spreading and strengthening Christianity among southern Slavs. There, in the 900s they constructed a new script for Slavic that was based on 24 capital Greek letters, and 19 letters for sounds specific to the Slavic language. This script became known as Cyrillic (кириллица) in honor of these pioneer linguists.
Over the time, the Cyrillic alphabet has seen many reforms. Some letters are eliminated, and some were added. For example, the first reformer of the Cyrillic Ivan Fyodorov, who was a printer and a publisher, eliminated the letters Е and С and many forms of the letter О. Also, at the end of the 18th century, the Russian writer and historian Nikolay Karamzin suggested to introduce the letter Ё. The letters Э and Й were officially added only in the 18th century. Most reforms saw the number of letters decrease and the simplicity of their inscription increase.
The Cyrillic alphabet went through a few reforms. In 1708, Peter the Great introduced lower case characters (before all letters were written with capital letters) and mandated the use of westernized letter forms, making the modern Cyrillic similar to the modern Latin font. The last reform was in 1918 when a few letters were lost and some spelling rules were added. The alphabet became the alphabet we know today.
Today Cyrillic is the third official script of the European Union after the Latin and Greek scripts. It is used in over 50 different languages, especially those of Slavic origin, mainly in Russia, Central Asia, and Eastern Europe such as Belarusian, Bulgarian, Macedonian, Serbian, Ukrainian and more.
On May 24th, Russia celebrates the Day of Slavic Written Language and Culture (День славянской письменности и культуры) in order to commemorate the creation of the Cyrillic alphabet by Saints Cyril and Methodius.
