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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.

The word "Yearly" is already in the title. Adding "annual" seems redundant.

sounds like post-hoc justification to me, you could have been confused other way

But here's a real solution (though I don't think it was a problem needing to be solved):

"FOSSY: the Free and Open Source Software Yearly conference, year 1"