Greek Christmas celebrations differ from traditional "western" celebrations in two ways: firstly, there is such a thing as a "Christmas boat" and secondly, Santa doesn't arrive on Christmas Day, or even Christmas Eve, but on New Year's Eve. Christmas itself isn't celebrated much, there are no special foods, cakes or traditions associated with it, apart from carol singing (but that takes place throughout the holiday season and isn't specifically linked to Christmas). New Year's Eve is a big family celebration, usually followed by partying until the early hours, clubbing or (in some cases) late night blogging ;-)
Νο one seems to know how the Christmas boat came about, but it's become extremely popular in recent years. Many Greeks decorate a boat as well as a tree at home, and many a city centre boast a Christmas boat alongside, or sometimes instead of, a Christmas tree.
The Greek "Santa Claus" isn't St Nicholas, but St Basil of Caesarea. He's dressed in red and white, looks exactly like the Santa we all know, but brings children their presents on January 1st. Tradition has it that presents are opened a few minutes into the new year, so you have to stay up until gone midnight on New Year's Eve to see what St Basil has put under the tree. No wonder it's the biggest party night of the year!
Mediterranean Kiwi has posted a more detailed account of Greek New Year traditions in her blog. You can read it here.
P.S. It's -6°C/21°F out there at the moment, but very dry. I wish it'd snow.