Solstice Week was a unique event in Pokémon GO that happened in June, 2017. This event celebrated the summer solstice in the Northern Hemisphere and winter solstice in the Southern Hemisphere.
During the event,
Fire- and Ice-type Pokémon had increased spawns. Pokémon like Charmander, Cyndaquil, Growlithe, Houndour, Ponyta, Swinub, Vulpix, and their evolutions appeared more frequently. Rare Pokémon like Sneasel, Magmar, Cloyster appeared as well.
Additionally, Trainers received
XP bonuses for successfully catching Pokémon with Nice, Great, and Excellent Throws, as well as Curveballs and First Throws. Hatching Eggs will also rewarded trainers with triple XP.
Following Pokémon had increased spawn rate
in the wild:
Shop special offer
There was special offer available in the
shop. The cost of Lucky Eggs was 50% off during the event. 
This event was leaked ten days early by Twitter user - Team Eeveelution, the same one who confirmed the Water Festival.
 An Italian and Chinese translation of the event's announcement stated that Gyms will be disabled starting on the 16 th of June.
 Curveballs would reward triple XP.
 The event was extended an extra day due to issues with the PTC account login.