There were the seemingly countless vigils and memorials, birthdays and anniversaries. And now the survivors of the Sandy Hook Elementary School mass shooting are set to graduate from high school — with memories of 20 classmates who were killed in the massacre.

“It’s a bittersweet moment,” says Matt Holden, who was a first grader at Sandy Hook Elementary on Dec. 14, 2012, and one of the 330 seniors who will graduate from Newtown High School on Wednesday night. “Graduation is supposed to be a very happy time. I mean, it’s probably the biggest milestone of our lives up until now. So we’re trying to keep it happy while knowing that there should be 20 of our classmates graduating who aren’t there.”

The school is planning a moment of silence at graduation for the 20 students and six educators who were killed that day:

It remains one of the deadliest school shootings in U.S. history.

Newtown High School’s graduating class will also honor the victims by wearing green and white ribbons, the colors of Sandy Hook School.

Holden was in his first-grade classroom making gingerbread houses with some parents when the shots rang out.

“I remember sitting in our cubbies, which were these little kind of, you know, holes in the wall area, for like 20 minutes,” he recalls. “I remember first walking out of the classroom and seeing an officer with his weapon drawn, which was the first kind of big sign for me as a six-year-old that something’s going on here.”

As they were evacuated, the children were told to cover their eyes. Then they made it outside.

“I remember that I saw some of the fourth graders crying,” he says. “And so I cried too, because they were the big kids, and if they were upset, we should be too.”

“The most striking thing for me on that day was when my mom came running up to me in the parking lot in absolute tears, hysterics. I mean, I’ve never seen her that broken down before,” Holden says. “And, you know, as a 6-year-old, seeing your parents like that, it’s not, it’s not normal.”

Earlier this week, on the eve of graduation, Newtown High School seniors took part in another tradition: paying a visit to their elementary school one last time. And for Sandy Hook survivors, it was just another bittersweet moment, and reminder of the lives lost on Dec. 14, 2012.

“When we did the walk through, we all took class photos,” Holden says. “And so when we did one for my kindergarten class, it was a class of probably like 15 or 16 kids. But this photo we took yesterday, there were only about eight kids in there because the other half of the class had passed.”

Roman Verna, another graduating senior at Newtown High, did not attend Sandy Elementary School. He went to Head O’Meadow Elementary across town. But he, too, has vivid memories of Dec. 14, 2012.

“I remember my first grade teacher, Mrs. Howard — we were doing reading in the front of her classroom and we turned off all the lights, locked the doors, closed the shades on the windows,” he recalls. “We had always done, you know, the evacuation drills and fire drills, and things like that.”

But this time there was no announcement that it was a drill.

“That’s when everybody I think knew something was different,” he says.

Verna did not know any of those students who were killed at Sandy Hook Elementary School that day, but he says he often wonders what they would’ve been like.

“I never got to meet them,” Verna says. “I could have been best friends with any of them, and that’s the part that really gets me.”

Jennifer Hubbard tries not to think about what her daughter Catherine would’ve been like as a teenager.

“I try not to go down that rabbit hole too far because I think that, at least for me, I would have the tendency to start, like, ‘Oh, she had so much potential that is gone,’” Hubbard says. “I feel like I don’t know what Catherine would have been.

Like, I’ve often thought maybe she would be a vet. Or maybe, she was kind of this quirky, whimsical kid, so I could see her being the kid that goes on safari and never comes home.”

Hubbard honors her daughter’s life with the founding 11 years ago of the Catherine Violet Hubbard Animal Sanctuary, a 34-acre property about a mile from the Sandy Hook school.

On June 8, Catherine would’ve turned 18. And Hubbard marked the day by hosting Catherine’s Butterfly Party, an annual pet adoption event in Newtown. This year’s Butterfly Party drew more than 15,000 people and resulted in close to 100 new pet adoptions.

To date, the sanctuary has helped find forever homes for over 500 animals and provided free veterinary support and pet food to the pets of 450 senior pet owners.

“At the end of the day, I like to think about just what Catherine wanted as a 6-year-old,” Hubbard says. “And it was pretty simple: She wanted animals that she was taking care of to know that she was kind and that she kept them safe.

“So when I start to think, ‘Oh, maybe she would do this, or, where would she be going to college right now,’ I then think, ‘Wow, all of the things that we’re doing at the sanctuary are kind of the grown-up version of Catherine.’”