Agreement of past participle

Hola Ann R.

Great question!

The reason is that in this case, "celebrado" is part of a present perfect subjunctive structure: "se haya celebrado." Although it may look like a passive construction, it actually functions more like a reflexive passive, and in this type of structure, the past participle does not agree with the noun.

In Spanish, past participle agreement only happens when the participle is used with "ser" in a true passive voice, like:

Here, "celebrada" agrees with "la competición" because it’s a classic passive.

But with structures like "se haya celebrado", the participle remains in its default form (masculine singular), regardless of the gender or number of the subject. 

I hope you find this explanation useful.

Saludos

Silvia