Arts occasions
Five days for poetry, theatre, dance, music and photography, with dates and ideas for messages that name an actual artist rather than praising art in general.
World Poetry Day on 21 March, World Theatre Day on 27 March, International Dance Day on 29 April, World Music Day on 21 June and World Photography Day on 19 August.
Most of them are run by UNESCO or by an international body in the field, and several publish a message each year from a working artist rather than an official, which is worth reading before you write anything.
Name someone
The trap on these days is writing about Art. Every post says the same thing about expression and the human spirit, and none of it is memorable.
The version that works names a person and a work. On World Poetry Day, quote four lines of a poet you actually read, in the language you read them in: Kabir, Mahadevi Varma, Kamala Das, Namdeo Dhasal, Agha Shahid Ali. On Dance Day, name the form and the dancer rather than "our rich classical traditions".
If you are writing to an artist you know, the most useful thing you can send is evidence that you paid attention: the line you remember, the show you saw, the photograph you still think about.
A practical note
World Music Day began in France as F锚te de la Musique, built around free public performances rather than posts, and several Indian cities now hold their own. On most of these days there is something on locally, and going to it is a better message than sending one.
For gratitude towards a teacher who taught you an art form, Teachers' Day and thank-you messages are the better fit.