If you are planning to visit Skiathos for holidays, you don't need to worry about the time of the year you'll be there since the island is equally beautiful all year round.
You would probably think that all Greek islands are like the Cyclades, very hot and dry. Visit Skiathos and you'll find out that this is not the case for every island in Greece!
Skiathos is located in the north-west of the Aegean Sea, very close to the mount Pelion, therefore its climate and weather is quite affected by the mountainous climate of the region.
In the summer, the average temperature is around 30°C during the day, but it can easily reach highs of 37°C in the hottest period, from mid-July till mid-August. However, the island is full of trees which make extreme heat bearable. During the summer months, there is often a refreshing breeze which also creates a cooling atmosphere and makes your stay even more enjoyable. At night you might need an extra cardigan or jacket, as sometimes the breeze is quite chilly. A special feature in the summer is the strong winds, the so-called Meltemia. These are typical of the Aegean and you will need to take them into account as far as clothing and boat trips are concerned, especially if you're visiting the island in July and August. If you like sunbathing and want to have a nice Mediterranean tan, then visiting Skiathos in the summer is the right time for you.
If you prefer to visit the island in spring, you' ll find yourself stunned by the beautiful colours of the flowers in the fields, the blossoms of the trees and the nice scents. Spring in Skiathos will make you change your mind about the stereotypical image of the treeless Greek islands. The weather is almost as lovely as in the summer with the average temperature being around 17-20°C during the daytime. Even though the temperature is high enough to enjoy the sun without getting sunburn, the water is still quite cold to go swimming contrary to the ideal temperature of the sea at the beginning of autumn, particularly in September. Nevertheless from October, the weather gradually changes and there is a significant decrease in temperature which ranges between 17-20°C, whereas in September the average temperature is still 25°C. The water is also getting colder and the sea much rougher.
As far as the rainfall on the island is concerned, from June-August you might experience light or heavy showers from time to time and more rarely a few storms, although they don't last long before the sun shines again. From March-May the weather might be rainy every now and then. The heaviest rainfall takes place from October and lasts during all winter, reaching 101-200mm.
The winter on the island is another story. It is much quieter and peaceful. The climate is mild, but the weather can be interchangeable and strong storms can strike the island. There have been years with light snowfall, especially in January and February, which made the landscape even more picturesque.