The prime minister and Gov. Gen. David Johnston will be there with their spouses, as will Quebec Premier Pauline Marois and Maine Gov. Paul LePage.
The prime minister will not deliver any speech inside or outside the church, but he and other guests might say a few words to the news cameras assembled outside.
Organizers have reserved 700 places in the 1,000-seat church for loved ones of the derailment victims. Remaining pew spots in the church have been set aside for locals, volunteers and dignitaries.
Two big-screen TVs will be set up outside the church and will broadcast the ceremony live.
The service will be held at 11 a.m. at Ste-Agnes Church and will be presided over by Luc Cyr, the archbishop of Sherbrooke.
An estimated 47 people were killed in the explosive July 6 train derailment, an event that has sparked several lawsuits, a police criminal investigation and a probe by federal transportation-safety officials.
The federal government has promised $60 million for emergency assistance and longer-term reconstruction help for the town. It has also revamped some rules on train transport, following the advice of the federal Transportation Safety Board.
It will be Harper's second visit to Lac-Megantic since the tragedy. In his first visit, he compared the devastated downtown to a "war zone."
The federal government had faced some pressure to provide details of its plans, after the provincial government moved days earlier in creating its own $60-million fund.
The clear target of local anger, however, has been the Montreal, Maine & Atlantic Railway.
The mayor of Quebec City added his voice to that condemnatory chorus Friday when he spoke to reporters during his visit to the damaged town.
With news crews assembled before him, Regis Labeaume delivered a string of epithets and accusations against the head of the MMA company — without naming Ed Burkhardt directly.
The pugnacious populist described MMA's boss as ''maybe the ugliest face of capitalism" and "the quintessential corporate bum."
Labeaume predicted that he would "suck" every available dollar out of the company, declare bankruptcy, and not lose a penny of his own in the swamp of impending legal bills and reconstruction projects.
"There's probably a firewall between him and the business," said the Quebec City mayor.
The mayor was asked what he thought of the federal government's aid response, given that it was slower than the provincial one, and he defended Ottawa's performance.