Question about visiting Heimaey

So I'm planning a trip for next year to Iceland with my parents, most likely in September. My mother really wants to go to the Beluga sanctuary. When I've searched through this subreddit about Heimaey though, I started seeing some comments of people warning others not to go during certain times, because it would be a "madhouse." Does anyone know what times that the island might be particularly crowded so I can try and plan to avoid it?