Πανηγύρι means “feast”, “fair”. It is a periodic festival either for religious or for commercial reasons.
It is a compound word (πάς + αγείρω). Πάς means “everybody” in ancient Greek. Αγείρω means “gather”. It has the same root as “αγορά” (agora), the central spot in ancient Greek city-states were people gathered for political speeches, shopping, etc.