Church services are due to be held around the nation, including at the ruined cathedral in Port-au-Prince.
There will also be a minute’s silence at 4.53pm – the exact moment when the 7.0 magnitude quake hit.
One year on, some 800,000 people are still living in temporary shelters.
As well as facing the huge task of rebuilding, which has barely begun, Haiti has had to cope with an ongoing cholera outbreak that has so far killed more than 3,500 people, according to government figures.
Political instability has also increased, following November’s disputed presidential election.
Commemorations have already been taking place in Haiti.
On Tuesday, President Rene Preval and other officials joined relatives of victims to lay wreaths at a mass grave on the outskirts of the capital.
Religious leaders also held prayers at the University of Notre Dame.
“Let the heart of the people never cease to beat for charity and liberty,” one priest said in his address.
Banks, schools and government offices were set to be closed on Wednesday for the day of national mourning.
Among those due to attend the service at the cathedral is former US President Bill Clinton, who on Tuesday told the BBC of his frustration at the slow pace of reconstruction in Haiti, a year after the earthquake that killed more than 250,000 people.
“No-one is more frustrated than I am that we haven’t done more,” Mr Clinton said during his visit to Port-au-Prince, in his capacity as UN envoy to Haiti.
But he said he was confident that the speed of reconstruction would pick up.
Several major aid agencies have questioned the effectiveness of the overall response to the earthquake. Medical charity MSF has pointed to a lack of co-ordination, Oxfam said donor countries had too often pursued their own aid priorities, while Merlin said the large number of NGOs has undermined Haiti’s own health service.
International donors last March pledged $2.01bn (£1.28bn) for the country’s long-term recovery but by the end of December, the amount disbursed totalled $1.28bn – or 63.6%.
One of the key problems is the continued political uncertainty. November’s election was widely denounced as flawed, with reports of fraud and intimidation at polling stations.
Violent protests broke out when the provisional results were announced in December.
The second round was due to take place on 16 January, but has been postponed until next month as there is still no agreement on which candidates will be taking part.
Provisional results put the former First Lady Mirlande Manigat in first place and the government party candidate Jude Celestin in second, just ahead of the pop star, Michel Martelly.
But Mr Martelly insists he won more votes than Mr Celestin and should be in the run-off.
A mission from the Organisation of American States, which was brought in to evaluate the result, is reported to have found in Mr Martelly’s favour, but this has not been confirmed.
Outgoing President Rene Preval said on Monday that he had not yet seen the report and has indicated he will only discuss it once the commemorations are concluded.