The tragedy took place when a runaway train carrying huge amounts of oil flew off a curb, setting off a massive inferno that devastated much of the Quebec town and killed up to 50 people.
Observances got under way late Friday, when about 100 people placed lit candles in the town's Saint Agnes church, not far from the site of the disaster.
In Montreal, about 200 people gathered at the Notre Dame Basilica to pay their respects.
"The loss of human life down there, it's crazy. Just crazy."
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Boucherville, about 18 kilometers east of Montreal, saw a procession of about 100 people carrying candles along the town's railway tracks.
Other ceremonies were planned today in Ottawa, Toronto, Winnipeg, and other Canadian cities and towns.
The death toll from the train disaster so far is at 28, with another 22 people missing and presumed dead.
The train, made up of 72 tanker cars loaded with crude oil, derailed in the early hours of July 6 in Lac-Megantic, a town of 6,000 near the Canada-US border.