Her sainthood date, on Sunday, falls on the eve of the 19th anniversary of her death.
She becomes a saint after Pope Francis approved a second miracle attributed to the nun's intercession in December.
She was an Albanian nun who won the Nobel Peace prize in 1979, and is best known for her work among the impoverished in the slums of Kolkata (formerly Calcutta), India.
The pontiff approved the healing of a Brazilian man with brain tumours who prayed to Mother Teresa.
Vatican spokesman Greg Burke said all 100,000 tickets have been distributed for Sunday's Mass but the crowd will probably be far greater, spilling into the main streets around St Peter's Square.
So far, 15 official delegations have confirmed their presence, 13 led by heads of state or government, and 600 journalists have been accredited.
The mass will be shown live on TV across the world.
Veronica Whitty volunteered with Mother Teresa in India for several months and is in Rome for her canonisation.
She told Premier: "She concentrated on you solely when you spoke to her. She gave the impression that you were the most important person to her at that moment.
"Despite being so busy she had all the time in the world to listen.
"Those are the qualities that I thought really stood out."
Andy Thonhauser from anti-persecution charity ADF international tells Premier Mother Teresa's admired in charities, governments and NGO's all over the world: "She didn't make a difference in how she spoke with people, no matter their rank.
"When she was addressing the Queen of England or mighty in the world she used the same words and she used the same stories as she did when she spoke to the poor or average people on the street."