WATERTOWN — When celebrating Vartavar, there is no such thing as a no-splash zone.

Everyone gets wet.

That’s the point of this ancient and joyous Armenian custom: to get drenched and, in turn, drench as many people as possible.

On Sunday morning, families, kids, and clergy filed out of the cross-shaped sanctuary at St. James Armenian Apostolic Church in Watertown and changed out of their Sunday best to wage a water fight of epic proportions and make their hometown live up to its name. In a nearby parking lot there were rows of buckets, hoses, and water blasters, three kiddie pools, and 2,000 water balloons.

“It’s a festival of life,” said Natasha Aljalian, the pastor’s wife. “The symbol of water has always been something that’s important to Armenians. It’s always been a sign of fertility, of life, of celebration.”

The ancient festival of Vartavar, or the Festival of Roses, eventually became celebrated on the feast of the transfiguration of Christ. It’s still remembered for the pagan feast it replaced, which marked the harvest and the bathing of the goddess Asdghig.

The celebration used to be gentler, said church deacon Herman Purutyan. People used to sprinkle each other with a bit of rose water. The modern-day festivities in Armenia, however, are no-holds-barred — a nationwide water fight. Fire trucks hose people down in the street. Water is thrown out of windows onto unsuspecting passersby — and into open car windows if drivers aren’t careful.

In Watertown Sunday, Arpi Tavil-Shatelyan of Somerville came armed with only a water bottle.

“It gives me memories of being in the Republic Square in Yerevan [the capital of Armenia] and just getting drenched,” Tavil-Shatelyan, 27, said. “Running around with my cousins where my grandparents live in their village and just maliciously coming up to my aunts and drenching them head to toe. There are only good memories associated with Vartavar.”

Samuel Chakmakjian, 23, of Arlington, wondered why it took so long for one of the local Armenian churches to embrace a Vartavar celebration. Watertown has one of the largest populations of Armenians in the nation.

“For me, it’s wonderful to have this kind of cultural expression in the open in Watertown,” Chakmakjian said. “We’ve been here for quite a while, and we’re sometimes a little bit too cautious about how we express our own traditions in public.”

Church leadership at St. James would like to see this mini-Vartavar grow into an annual tradition that acknowledges the fun and remembers the feast day.

By early afternoon, even adults and neighbors jumped into the fray, grabbing hoses, water blasters, and water balloons before chasing anyone that looked dry. Kids squealed as they rode the water slide and tried to avoid streams of cold water. Elders sat in the shade out of the range of hoses, smiling at the antics, while an ice cream truck gave out free Popsicles.

With a mischievous smile, Thatcher Simmons, 8, of Belmont, filled up the biggest bucket he could find.

“I need to fill her up,” Simmons said. “So I can soak everyone.”

Water gun in hand, Kevork Atinizian, 18, waited out his brother, Antranig, 15, who was hiding inside the church trying not to get wet. On this day, being dry was not an option. It was a day to remember the joy of living, of surprising others, and the innocence of childhood pranks.

“I grew up going to an Armenian camp, and we did this every summer,” said Alexis Demirjian, 40, of Belmont, moments before her 10-year-old daughter Meline, poured water on her and her 2-year-old son, Aram.

“My husband asked me, ‘Why did you wear a bathing suit,’ ” she said laughing. “Because I knew this was going to happen.”

The Rev. Arakel Aljalian remembers coming home after sundown last year on the day before Vartavar. He’d just finished performing a baptism. His kids were hidden behind the fence, waiting with water guns and water balloons.

On Sunday, the pastor didn’t even remove his vestments as he began soaking congregants. He’d told his wife to make sure to save him a hose.

“We figured it was summer, it was hot, and it’s something we all usually do in our own homes,” said Natasha Aljalian. “No Armenian is surprised if they’re hit with water today, and nobody will get upset because it’s just really about celebrating life and being happy.”