For north shore Kauai, if you are not looking for big surf) it is most definitely late May early June, September, early October.
Water calm , weather great, mangos in May. Don’t go when schools out for summer, it’s crowded.
A little more rain in May then September but usually just a short squall. Late October starts big surf on the North Shore. Water can be treacherous if you aren’t super experienced. The South side is calmer then.
Went in April once, 5 solid days of rain on the north shore.