In general, the islands “close for winter” at the end of October (beginning of November) and, depending on the island, the season is either extended or is a bit shorter.
Greek islands which are smaller and less touristic have a shorter season, larger islands like Crete, Corfu and Santorini have extended seasons sometimes up to full year. Hotels have different opening/closing dates but the general rule for islands is that they open in April and close at end October. It should be noted that islands which have shorter holiday season (usually the smaller or less know of the Cyclades) are still very quiet in April and best visited from May onwards.
Obviously exceptions exist. It is not only the hotels opening though, it is the general the ambiance and feel of the island which is affected when most of the hotels and restaurants are closed – for example, visiting Mykonos before season starts does not provide a good travel experience as most of the island (all travel related products) are not operating. The island is targeted towards summer and beach experiences. So it is not only the actual opening/closing of hotels but more of what the guests will experience while on the island.
An example of this is the island of Crete which although having a 12 month travel season; in the winter the trip itinerary and daily experiences are extremely different from the summer.
It is always good to remember that although Greece is known for the amazing summer days on the beaches, it also has a winter season which has lower temperatures and even snow on the islands.
12 months of travel in Greece
Greece has exceptional winter products including cultural and experiential trips on the mainland; usually these trips do not include visits to the smaller Greek islands.
Larger islands, like Crete, could be visited as a winter destination with longer stays on the island (usually of more that 4 days).
Greece has such rich culture and beautiful nature allowing for exceptional trips in the winter. The Greek mainland has wonderful boutique hotels full of character and amazing locations.