The Summer Solstice is a day that usually falls on June 20 on the human calendar. It is considered the longest day of the year.
In the EMPATH: The Luckiest Smurf story series, the Smurfs usually celebrate the Summer Solstice with a picnic or a beach party. As mentioned by Empath in his personal journal, the one time that the Smurfs attempted to celebrate the Summer Solstice, the day had started off rainy. Handy decided to deal with the problem by creating the weather-smurfing machine that would allow Smurfs to control the weather. This led to the events that took place in the cartoon show episode "Foul Weather Smurf".
With the creation of the Imaginarium, however, the Smurfs could now hold a Summer Solstice celebration on a beach setting regardless of the weather, which in the story "Polaris' Fantasy" turned out to be rainy.
Expanded Animated Universe
In the Expanded Animated Universe, a celebration called the Summer Solstice Extravaganza is held annually around that time of year by the Royal Gnoman Family of the North Palace in "The Gnome Princess" of Season 10. Princess Glacia had her parents invite King Gordon and his son, Prince Pyro with whom she had grown close. At the party, she admitted having strong feelings for him and he reciprocated them, and so they received their parents' blessings to date from then on.