Average daily temperatures now are 66-81°F/19-27°C and over half of each day (thirteen hours, on average) is sunshine. Rain has also decreased so much as to be virtually insignificant, with 0.2 inches/ 5mm generally falling this month. Seawater has warmed up too (21°FC/70°F), although strangely, because of how long the sun takes to truly warm the water, if you are swimming off one of the island's beautiful beaches in June you will be no warmer than you would in October.
The island can be affected by the Meltemi this month, a wind from the north that blows persistently for up to several days at a time and is liable to make the air feel cooler: good because it injects some freshness into an otherwise sweltering summer but bad because it can make the sea rough.
For more on weather this month, see Greece in June: Travel Tips, Weather, & More.
Crowds & Costs
The trade-off for getting all this glorious weather is how many other visitors are coming to Santorini for exactly the same reason. From now for the next three months, this is high season, and costs for everything on the island will be as high as they get all year. This is particularly noticeable at the hotels.
Everywhere, it will be significantly more crowded even when compared to last month. It will take you longer to wander down Fira or Oia's time-lost streets, your coveted restaurant table with the perfect sea view may be taken (along, indeed, with every other available seat there) and lots of tourists wanting the same shot as you will be straying into your photo frame. That said, venture far enough from the main tourist sights and you can still find relative peace, even now.
Where to Go
Get out your beach towel and head to one of Santorini's sublime stretches of sand. The beaches are mostly black sand and small pebbles, but this doesn't detract from their beauty. The majority of beaches are on the east side of the island, away from the west coast which forms the caldera.
Kamari is a popular black sand beach with a hotel sitting directly on the sand; other sandy beaches with hotels close by include Perissa and Perivolos. A visit to the famously striking and aptly-named Red Beach, comprised of small red stones and with beautiful crystalline water, should be on your itinerary too, as should the long and lovely Baxedes Beach near Oia.
Traveling out to the far southwestern end of the island, beyond the archaeological site of Akrotiri to Akrotiri Lighthouse, is one of several ways to lose many of the crowds. Here, lesser-visited beaches include Kambia Beach, White Beach and Black Beach.
What to Do
In the beautiful warm weather June offers, sunning yourself on the beach or swimming in the island's famously clear waters have to be the top choices of activity. You could also indulge in a spot of sea kayaking or even take a cruise around the island's coastline and huge volcanic caldera.
If it's a break from the water you crave, explore the island's fantastic archaeological sites, such as the ancient Minoan city of Akrotiri, or cool off with a visit to one of Santorini's wineries, many of which are open for tours and tastings. One of the best wineries to visit is Santo Wines near Pyrgos.
Feast Day/ Holiday of St. Pnevmatos (Holy Spirit) This is a 3-Day weekend based around the movable feast day of this saint, and becomes a big vacation for locals, who often leave the big mainland cities for the islands at this time. All the islands are full as are the ferries going there on Thursday/Friday and returning on Sunday/Monday/Tuesday. The weekend it falls on depends on when Orthodox Easter was.