Our Lady of Madhu church, in a remote wildlife reserve in the island's north, was on the front line of a decades-long civil war between Sri Lanka's army and separatist guerrillas seeking an independent homeland for the Tamil minority.
The church is a pilgrimage destination for Catholics from both the Tamil and the majority Sinhalese communities, making it a rare symbol of reconciliation in a country riven by ethnic and religious divisions.
Priest Emilianuspillai Santhiapillai expects at least half a million people to attend on Wednesday when Pope Francis will fly in by helicopter for a religious service.
The first papal visit to the island in two decades comes after the surprise ousting of former president Mahinda Rajapakse, who oversaw the army's violent suppression of the Tamil Tiger rebels in 2009 and faced criticism for failing to bring about post-war reconciliation.
"We needed change," said 51-year-old construction engineer Palitha Rodrigo, a Sinhalese Catholic who travelled 300 kilometres (180 miles) from Colombo to attend mass at the church.
"He (Rajapakse) could have done more things for the Tamils. We don't want to have fighting in Sri Lanka."
Sri Lanka is a majority-Buddhist country, but it has a sizeable Catholic minority, accounting for around six percent of its 20 million people. Hindus represent around 13 percent and are mostly Tamil.
"Catholics, Hindus, Buddhists and Muslims all come here for the harmonious and peaceful atmosphere.
