How did I miss this one?

April 26, which is the feast day of Stephen of Perm, is celebrated as Old Permic Alphabet Day.
Not mentioned by Crystal. A passing reference by Geoffrey Sampson, in a LinguistList review of a book on Uralic languages, led me to it.
It was invented in 1372 by the said Saint, in order to write (Old) Komi (aka Old Zyrian), a Permic (Uralic) language (making it one of the earliest scripts used in that family). It is a rather radical re-shaping of Cyrillic, with some elements from other sources. Its use continued for about three centuries, after which it was replaced by more normal Cyrillic.
All this and more at:
https://en.wikipedia.org/wiki/Old_Permic_script
We've already mentioned Hangul Day, celebrated in Korea. Any other holidays dedicated to particular scripts?