An amazing opportunity awaits genealogists today and for the next two days, ending at 10:59 p.m. on Tuesday. During the three days, family history researchers have free access to online databases of the New England Historic Genealogical Society.

The society is the oldest and largest genealogical society in the United States. It was founded in 1845 and has its headquarters at Boston.

Through the society’s website, users have free use of the world’s largest collection of authenticated Pilgrim databases. Last year, over 2 million people searched for their ancestors at the site.

Normally, users must be members of the society. During the three days, however, genealogists can register as a guest and have free access to the databases and other information that are online at AmericanAncestors.org/July. The site includes over 1.4 billion searchable names from around the world.

Nine of the databases are about the Great Migration, the time period between 1620 and 1640. During those two decades, over 80,000 people cut their ties with the Church of England and moved to other areas. The separatists became known as Puritans.

Some Puritans moved to Ireland, some to the West Indies and some to the Netherlands. Over 20,000 of the Puritans moved to New England. (By 1640, about one-tenth of congregational members returned to England, and one-third of clergymen returned.)

The nine databases relating to the Great Migration to New England are part of a project called “Mayflower Families Fifth Generation Descendants, 1700-1800.” The earliest Puritans to New England, who were also known as Pilgrims, founded the Plymouth Colony.

The Mayflower databases, which have half a million names, are the result of a partnership between NEGHS and the General Society of Mayflower Descendants.

One of the newest set of databases to be added to the website has details from 100 parishes of the Roman Catholic Archdiocese of Boston.

That database, which spans the years of 1789 to 1900, includes records related to baptism, confirmation, communion, marriage and anointing of the sick.

These databases at the NEHGS site, as well as online historical content from the NEHGS library and archives, offer an amazing treasury of details that might break down the walls that you have encountered during your search of New England ancestors.