Nothing has been confirmed yet, but if the trip does go ahead, it
would be important, not just because it would mark Francis’ first
official visit to the US, but also in terms of the defense of the
traditional family in light of a growing international pressure for the
legalization of same-sex marriage
Pope Francis has apparently expressed an intention to visit the United States in September 2015, for the 8th edition of the World Meeting of Families. 
This is according to Vatican sources who spoke to NCR.
They stressed, however, that nothing is official yet and that it is too early to give any certain dates.
The World Meeting of Families is held every three years in different parts of the world and was first launched by John Paul II in 1994.
In February 2013, shortly before the resignation of Pope Benedict XVI, the Vatican announced that the next event would take place in Philadelphia between 22-27 September 2015, John Allen recalls in his article for NCR.
The family has been one of the main focuses of Francis’ pontificate and is the subject of the next October’s meeting of the Synod of Bishops.
Allen writes that the Pope’s visit is uncertain because Popes have not always attended the World Meeting of Families in the past but there is speculation that Francis could combine a visit to Philadelphia with a trip to New York to address the U.N at the General Assembly of the United Nations which usually takes place in September.
This week, two senior Vatican officials told NCR on background that Francis has expressed a desire to attend the meeting in Philadelphia but an official from the Philadelphia archdiocese said "these trips are never 'set' until they're official" and that church officials in Philadelphia have had "no official indication." "We hope, but we don't know," he said.
This trip would be highly significant were it to take place, not just because it would be Francis' first papal visit to the United States, “but because defense of the traditional family, defined as a union between a man and a woman and open to children, has been a major concern for the Catholic church both in the United States and in other parts of the world, especially in light of a growing push for legal recognition of same-sex marriage,” Allen points out.