How to mark a section as 'only for certain performances'

I'm scoring a religious piece that has an extra 6 measures in the middle that only happen when performed on the Sabbath. What is the convention for dealing with this? Is it just a text note when those measures show up, with perhaps a different font color? Can I bracket them in some way? Is there a musical notation I can use? I've attached a picture - I created red brackets here using a picture editing software, not using musescore itself.