The first public hearing in a long called for public inquiry into the Omagh bomb will start on Tuesday.
The inquiry, led by chairman Lord Turnbull, is examining whether the atrocity could reasonably have been prevented by UK authorities.
Twenty nine people, including a woman pregnant with twins, were killed in the Real IRA bomb attack which devastated the Co Tyrone town on August 15th, 1998.
Some of the bereaved families have called for decades for a public inquiry into alleged security failings.
Michael Gallagher, whose son Aiden was killed in the atrocity, said the hearing will be the first time since 1998 that all of the bereaved families, including those caught up in the blast from Spain, will come together.
The inquiry, ordered by former Northern Ireland Secretary Chris Heaton-Harris last year, is examining alleged security failings that led a High Court judge to conclude the attack could plausibly have been prevented.
It was ordered in response to a court judgment that directed the UK government to establish some form of investigation.
While having no jurisdiction to order the Irish Government to act on the matter, the judge urged authorities in Dublin to establish their own investigation in light of his findings.
The Irish Government earlier this month formally agreed to provide assistance to the inquiry.
Survivors are also expected to gather at the Strule Arts Centre in Omagh for the hearing which will include opening remarks from the chair and counsel to the chair Paul Greaney KC.
Commemorative and personal statements are set to be made during inquiry hearings next January.