Being situated in the northern part of Greece, Samothrace generally has a colder climate than that of the other islands. The winters are mild and rainy but sometimes you can see winter on the top of the mountain or even in the settlements on the shores. The springs are sunny and the summers hot.
The geographical position and the characteristics that the Saos Mountain altitude gives generates variations in the microclimate of the various settlements on the island.
The best months for a a holiday are May (when the nature blossoms), June, July-August (although sometimes quite hot) and the beginning of September.
The best time for a holiday?
It depends on what you like doing. Every season provides a different experience.
The spring is beautiful for the craziness and exuberance of nature, but most probably the sea water won't be worm enough for swimming. It is a good period for trying local authentic experiences.
In the summer the island becomes crowded and the prices are higher. There are a lot of concerts at the beach bar in Therma, the tavernas and the coffee shops are busy and everything is full of life.
The autumn is special for the peacefulness and for the changing colors of the vegetation. The prices are lower than in high season, the sun doesn't burn so hot and the waters are still good enough for swimming. It is also a good time for trekking. You should though take into consideration that, even if the weather is usually quite good until Octomber, many villas, shops, bars and cafeterias already close after 15th-20th of September, waiting for the next season to come.
There are little tourists who dare to come to Samothrace in the winter, but these are the real lovers of the island, who wish to discover it in all its colors and aspects. Therma is almost abandoned, as the inhabitants move to other settlements and the accommodation possibilities are limited. Quite often the ferry doesn't work because of the weather conditions and the island can remain isolated for days.
You can check the weather here.