Myositis, which includes dermatomyositis, polymyositis and inclusion body myositis, is one of the rarest autoimmune conditions, and affects 75,000 adults and children in the United States alone. I personally don't really know if this is rare or not! I have searched around the net though and not found much mention of Myositis Awareness Day. (Maybe that is because it is rare??)
Here are the links for what I found that is happening for Myositis week in:
USA National Myositis Awareness Day
Do you have any other links to add?