Question

Approximately when was European music first written down​

Answer 1

By the middle of the 9th century, however, a form of neumatic notation began to develop in monasteries in Europe as a mnemonic device for Gregorian chant, using symbols known as neumes; the earliest surviving musical notation of this type is in the Musica disciplina of Aurelian of Réôme, from about 850.

Answer 2

Music was actually written down as far back as the 6th Century B.C. in ancient Greece and Rome. The writing in this time was a bit more of a reminder on how to sing the songs rather than precise notation but it was certainly the beginning of writing down music in a digestible form.