The San Juan festival takes its inspiration from the biblical character, St. John the Baptist, who converted people to Christ by blessing them with water, thus the use of water for merry-making. From the city mayor to street urchins, San Juan residents participate in an all-out drenching using water bottles, water bombs, water pistols or the lowly tabo (dipper). Even fire hydrants and fire trucks are fair used. For common passers-by or jeepney passengers whose rides happen to pass by the city on this day, there's no other way to keep dry except to bring extra sets of clothes secured in plastic bags so they can change upon reaching their destinations.
and closer to home, the Mud Festival (Taong Patik) is also celebrated on St. John's day.
Nueva Ecija journal article
In the Philippines, as in other Catholic countries, June 24 is observed as the feast day of Saint John the Baptist. The day offers an excuse to engage in revelry such as dousing water on unwary people.
In Aliaga, Nueva Ecija, in Barangay Bibiclat, hundreds of devotees of the village's patron saint, John the Baptist, transform themselves into "mud people" -- literally "taong putik." The locals call the ritual Pagsa-San Juan....
Nobody knows exactly when the Taong Putik Festival started. One legend says the image of the patron saint which was brought to Bibiclat by early Ilocano settlers, helped in driving away poisonous snakes from the village... Another legend says that when Japanese soldiers during World War II were about to execute all the men in the village in retaliation for the death of 13 fellow soldiers, it rained so hard that the male villagers had to be herded into the church to seek shelter. After a while, the Japanese soldiers had a change of mind and set their captives free. The residents attributed this to a miracle of Saint John the Baptist, and vowed to pay homage to him on his feast day by wearing costumes patterned after his attire -- this time, using native materials.
Al Jezeerah article