there is probably some issue of wanting to make sure contracts are signed and performers are confirmed to avoid "false advertising" or misleading info or need for apology later...

but it would always seem that TBA (to be announced) could be the
disclaimer.

here is one web site for a music festival in the U.S. that is well
run, well organized, and well enjoyed...www.strawberrymusic.com

this event happens twice each year