Santorini, officially known as "Thira" in Greece, is an island sitting off the southern portions of the Aegean Sea, around 200km from the southeastern parts of mainland Greece. Part of what used to be a volcanic caldera, Santorini is the largest island forming the archipelago of Santorini, which completes the group of islands, Cyclades. If you're headed for Santorini, best to keep in mind that the municipality is made up of both inhabited (Santorini and Theresia) and uninhabited islands (Christiana, Aspronisi, Palaia Kameni, and Nea Kameni). If you're headed for Santorini, best to keep in mind as well what the Santorini weather is like.
The climate in the island of Santorini is very typical of what the Mediterranean has to deal with: generally long periods of sunshine through most of the year, with relatively dry and warm summers and rainy and mild winters. Weather in Santorini can be subdivided broadly into two seasons: the warm, dry season from April to October and the cold, rainy season starting in November and ending in March. It's not uncommon to have long periods of time with consecutive rainy days, even in the peak of winter, and yet Santorini enjoys skies that don't remain cloudy for days in a row, much like what other regions in the world do. Tourist season is generally from April to September during the warm, dry season so visitors to the island usually enjoy stable weather, with clear skies, bright sunshine, and zero rainfall. Even though the season is warm and dry, the evenings do cool down enough to require a jacket or sweater for those going out after night fall. If you think you're going to be finding yourself in Santorini during this time, best to keep this reminder in mind. Apart from the island, Santorini weather is only experienced in Anafi in the whole of Europe.
Knowing the weather in your destination is important because this dictates what you can and will generally enjoy when you get there. And besides convenience, knowing the weather is also a matter of ensuring that you are safe. It's a good thing that weather can more or less be predicted these days so you have a means of checking how things are in your destination. You can easily do so by going online. Depending on what will work for you, you can generally check how the weather is in Santorini by day, although there are also estimates for weeks and the entire month.