San Juan Gran Canaria celebrations take place at night (23rd June) on Noche de San Juan with huge fireworks displays, bonfires, live music and parties on the beach.
June 24th is the day of San Juan so all of the above is on the eve normally with the fireworks being the climax of the evening at midnight just before everyone jumps into the sea, to have a cleansing swim.

There are quite a few traditions surrounding the night of San Juan and they differ from region to region. The fire is supposed to burn away all of the badness and the swim is to cleanse the soul. Much like New Years Eve, Noche de San Juan is a time to start fresh and celebrate the start of Summer.
In terms of fires and fireworks it is much like the UK's Bonfire Night (sans burning effigy of Guy Fawkes). Families gather in groups on the beach with picnic tables, sandwiches and snacks for the night while they camp out until 4 or 5 in the morning. This also marks the beginning of the school Summer holidays, as the children break up in the days leading up to San Juan.

It's the most magical night of the year and a time for new beginnings. San Juan celebrations usually take place along the coast, with a huge party on Playa de las Canteras in Las Palmas. Each location will usually have grills or ‘asaderos', music and plenty of dancing.
If you are in the south of Gran Canaria for this special evening then head to Anexo I in Playa del Ingles. They have a great drum circle every year that really sets the atmosphere, plus a stage with live music.
So if you know somebody called Juan or Juanita, on June 24th you can tell then Felicidades as this is their special day, on par with birthdays.