The two girls, ages 5 and 4, were asleep with their parents, Tara and Heath Carey, when a natural gas explosion ripped through their apartment building at 65 Main St. in Hopkinton last year.

Violet and Iris were trapped in the rubble and later died. Several people were injured.

In e-mails and telephone interviews, the girls' parents said they decided to mark the one-year anniversary to honor their daughters' memories.

"It is a very emotional time when we are face-to-face with the actual day of their passing," they wrote in an e-mail. "We would love people to stop by and help us show that Violet and Iris' light will always shine. They will never be forgotten, and are so forever loved and sadly missed."

The vigil will begin at 7 p.m., at the property that has since become a memorial to the two girls.

In addition to the vigil, family members will gather today at the Vernon Grove Cemetery in Milford for a private blessing of the girls' headstone.

Following the blessing, the family will release Painted Lady and Monarch butterflies.

"We know that is something they would love to have happen," Tara and Heath wrote.

In a telephone interview yesterday, Heath Carey said the family is still focused on planning the memorials, but said emotions will be tough to contain today.

"I think it's really catching up with us," he said. "I think right now we're just very caught up in making sure everything gets together. I really don't think it's going to hit until we're actually there and everything is going on."

Investigators are still probing the cause of the explosion, and have recently focused their attention on a fitting that connected the gas line from the street to the house.