The private gathering on the eve of the anniversary of the atrocity comes against the backdrop of angry recriminations by many of the bereaved.
The private gathering on the eve of the anniversary of the atrocity comes against the backdrop of angry recriminations by many of the bereaved, who say official incompetence and neglect since the assault have inflicted fresh wounds.
Last December 19 at 8:02 pm, Anis Amri, a 24-year-old Tunisian who had failed to obtain asylum, rammed a stolen truck into crowds at the market on the Breitscheidplatz, a popular destination for Berliners and tourists alike.
The victims came from Germany as well as countries including Israel, Italy, the Czech Republic and Ukraine.
More than 70 people were injured in the attack, the deadliest ever carried out in Germany.
The Islamic State group claimed responsibility the next day, and Amri was shot and killed four days later by police in Italy, where he had previously lived.
Although Merkel has visited the scene of the attack at least four times, including once to the reopened Christmas market last week, it will be her first face-to-face talks with relatives of the victims, some of whom have accused her of ignoring their needs and concerns.
In a wrenching open letter to the chancellor this month, before the meeting was announced, several family members condemned her "political inaction" and accused her of failing to reach out to them.
"Almost a year after the attack, we note that you have not shared your condolences with us either in person or in writing," the letter said.
"In our opinion, this means that you are not living up to the responsibilities of your office."
A government-commissioned report released last week identified a litany of shortcomings in the response to the tragedy.
Some relatives desperately searching for their loved ones were told only three days after the attack that a family member had perished, even though they could have been given early warning through facial identification.
Others were sent "bills for autopsies -- including warnings for late payment, I didn't want to believe it, but I had such a letter in hand," said the author of the report, Kurt Beck.
"Such experiences should never be repeated," he said, adding that Germany "was not prepared" to deal with the attack's aftermath.
The government has paid out 1.6 million euros ($1.9 million) in compensation to the wounded and victims' families.
Justice Minister Heiko Maas pledged that Berlin would take action "to ensure that when something so terrible happens that the relatives of victims are taken care of as well as possible."
"We have learned from our mistakes," he said.
Another factor keeping the wounds raw has been steady leaks in the press about administrative gaffes and missteps leading up to the attack.
Amri, who arrived in Germany in the summer of 2015, at the height of the refugee influx, registered under several different identities. Media reports citing the investigation have said that he plotted the attack from the start.
Authorities knew him to be an Islamist extremist and drug dealer whose asylum claim had been rejected and who was being intermittently monitored by police.
But Amri was never deported or arrested.
Israeli tourist Rami Elyakim, 64, who lost Dalia, his wife of four decades, in the attack, said he remembered only drinking mulled wine together at the market.
Elyakim, who sustained broken bones throughout his body and still has difficulty moving, said that living in Israel he and his family had grown used to attacks, but they did not expect terror would strike them in Berlin.
"We thought Germany was safe," he told the Bild newspaper. "In Israel no one who was planning something like this would walk around free. Maybe the Germans are naive."
On the anniversary itself Tuesday, the Christmas market will be closed for the day so the families and first responders who tended to victims can attend a memorial ceremony in the church on the same square.
Merkel, President Frank-Walter Steinmeier and Berlin Mayor Michael Mueller will inaugurate a memorial -- a 14-metre (46-foot) golden crack in the ground engraved with the victims' names.
In the afternoon, the site will open to the public for those wishing to pay their respects and join in a prayer for peace at dusk.
At the exact time of the attack, the church's bells will chime above a sea of lighted candles.