Where are the mermaids?

My sim has become good friends with Nalani and she now swims in the ocean with her mermaid tail, she's so pretty! I have not found any mermaids otherwise yet, but once when my sim went thru the secret cave she came out flirty from hearing the siren song from a mermaid there. Wish I had known who it was, thats so cool! I can't seem to find the events around town, where are those? And is it at night only? I want to be sure to catch one and my sim is 1 week away from the whole Spring season, which she started in.