We sailed Croatia in May 2005 and had a mixture of fine, sunny weather and occasional periods of heavy rain and high winds. A week long charter
is risky at that time of year since you might hit a period of the latter rather than the former. There's no guarantee, but Kos and the Dodecanese tend to get more settled weather in early May; we've cruised that area several times in early season and rarely encountered bad conditions. From Lavrion, however, you've got a long sail to get anywhere interesting and if the weather turns while you're in the Cyclades
, for example, it's a nightmare getting back. I you opt for Lavrion, I'd advise staying in the Saronic Gulf if you're only taking a week.