Inaugurating a Hindu temple on this particular site is polarizing event within India.
Modi's Hindu-nationalist Bharatiya Janata Party (BJP) is hailing the move and has set up lavish celebrations across much of the country, not just in Ayodhya near the northern border to Nepal.
But the temple is being built on land where a mosque had stood for centuries before being torn down by Hindu nationalists in the 1990s, sparking violent protests. The prime minister has tried to argue that the event is a bid for reconciliation.
"The Lord has made me an instrument to represent all the people of India," Modi said earlier in January, announcing that he planned to undergo 11 days of ritual fasting ahead of Monday's "auspicious" opening.
No expense spared, as election campaigns loom
The construction of the grand Ram temple cost around €166 million (roughly $181 million). The opening is significant for India and for the BJP as it's emblematic of the increasing influence of Hindu-nationalist politics under Modi's government.
The inauguration ceremony of the Ram temple is set to begin at just after noon local time (around 0700 GMT/UTC) on Monday with a black stone idol of Lord Ram to be consecrated as the deity of the temple.
Thousands of Hindu devotees danced in packed streets on Sunday in Ayodhya as blaring loudspeakers played hymns and religious songs.
More than 2,500 musicians are expected to perform during Monday's ceremony, which will be attended by many prominent personalities including sports and Bollywood celebrities.
The celebrations are not limited to Ayodhya. In the capital city of Delhi, saffron flags with pictures of Lord Ram and the temple can be seen atop vehicles, shops, houses and offices.
Controversy surrounding the Ram Temple
The 50-meter high temple has been built on the land where a mosque stood for centuries before it was demolished in 1992 by Hindu fanatics associated with Modi's Bharatiya Janta Party (BJP).
Hindus meanwhile claim that the Babri mosque was in turn built over Lord Ram's birthplace by a 16th-century Muslim emperor.
The demolition of the Babri mosque triggered communal riots that claimed more than 2,000 lives — most of them Muslims. It also helped pave the way for more sectarian politics, marking a shift from India's founding priniciple of a secular political order.
The construction of the temple was even challenged, albeit unsuccessfully, in India's top court in 2019.