Semi-formal (see above photo). Jacket and tie optional. There will not be a whole lot of walking but please wear shoes that will be suitable for stairs, cobblestone, sand, etc. The whole wedding will be outdoors and the weather that time of year is typically mid 60s - high 70s. I recommend bringing a layer for when the sun goes down.
Our recommendation is September 16th - September 20th. We arrive in Skiathos September 14th to tend to some last minute wedding details. The fun begins Monday, September 16th with the welcome party. Tuesday, we marry! Wednesday and Thursday we will be celebrating, island hopping, etc. and we leave Friday, September 20th. We will be in Athens Friday night for anyone who wants to meet up.
All you will need is a valid passport. Make sure your passport has at least six months of validity past your departure date.
Most places accept credit cards. However, you should have some euros on hand for small expenses and tips. ATMs are widely available, but can charge a high exchange rate. We would recommend visiting your local bank a few weeks before your trip and arranging for currency exchange. Here is some additional info: https://www.nerdwallet.com/article/banking/where-to-exchange-currency-without-paying-huge-fees#:~:text=Though%20there%20may%20be%20a,pick%20up%20at%20a%20branch.
Wifi is available in every hotel and many restaurants, bars, etc. If you want service beyond that the best option is to purchase a local SIM card. You can purchase a SIM card at the airport or in Athens/Skiathos at a Vodafone. We paid $20 for a month of unlimited data and 200 minutes.
Your presence is gift enough. We have a small registry at the request of some friends/family who won't be able to make the trip, but we truly don't expect any gifts. https://www.zola.com/registry/leavelltrakaswedding