We've been during the last week of April, some tavernas open, the sea is warm enough to swim in but nothing like September, and one year - 2009/10?, can't remember exactly - there had been snow on the beach in late March, and wandering down for lunch with shorts/tee-shirt was definitely a mistake!
So, pot-luck, I suspect. That said, if wild-flowers and walking are your thing, then April is grand.