As if fosdem isn't annual. It would possibly confuse me more tbh.
"The first instance of" or so might be more clear but, yeah, the catchiness degrades with every attempt at making it clearer. One might even be lead to think that "Free and Open Source Software Yearly" is not an amazingly unambiguous name to use in a world with fosdem, t-dose, and probably others.
Maybe it would be more clear as:
"The first annual Free and Open Source Software Yearly conference"
Instead of:
"The first Free and Open Source Software Yearly conference"
Putting "annual" in the title makes that meaning / context more clear to me.