The Marian Days (Vietnamese: Đại hội Thánh Mẫu, officially Ngày Thánh Mẫu) is the main festival and pilgrimage for Vietnamese American Roman Catholics. The annual event in honor of the Blessed Virgin Mary has taken place the first weekend in August since 1978 on the 28-acre (110,000 m) campus of the Congregation of the Mother of the Redeemer (CRM) in Carthage, Missouri. Tens of thousands of attendees come from throughout the United States, while non-Vietnamese locals and some visitors from Canada, Vietnam, and Australia also attend.

On April 30, 1975, 185 clergy – about half of the Congregation – left Vietnam as boat people just before the Fall of Saigon. They arrived in the United States at Fort Chaffee and other Operation New Arrivals refugee camps. Cardinal Bernard Francis Law, then Bishop of Springfield–Cape Girardeau, sponsored the priests and brothers, inviting them to rent a vacant Oblates of Mary Immaculate seminary, Our Lady of the Ozarks College, for a nominal price of $1, to use as their U.S. monastery. Between June 30 and September 3, 1975, nine priests, 154 brothers, and four novices arrived in Carthage, a predominantly Protestant town. The Overseas Congregation of the Mother Coredemptrix received formal recognition from the Congregation for the Evangelization of Peoples on September 16, 1975, and the congregation's Assumption Province (Vietnamese: Tỉnh Dòng Đồng Công Hoa Kỳ) was established on October 25, 1980, with Very Rev. Barnabus Maria Nguyễn Đức Kiên as the provincial. The Holy See gave the province a mission to minister to the Vietnamese American community.

The Congregation organized the inaugural Marian Day at its U.S. headquarters in 1978, in celebration of the Immaculate Heart of Mary. Around 1,500 Vietnamese Catholics from the Carthage area participated in the one-day retreat.

Ordinarily, Marian Days takes place without major incidents. The Carthage Police Department and event organizers enforce rules against indecency and drug use. Gang members are banned from the event, after two gangs killed a man during a fight in 2003. In 2008, 17 pilgrims died in a bus crash en route from Houston to Carthage.

Around 60,000 attended the 34th annual Marian Days, August 4–7, 2011. Presiders included Bishop Johnston and Bishop Emeritus Leibrecht of Springfield–Cape Girardeau, the local diocese; Archbishop Gregory Aymond of New Orleans, Bishop Stephanus Tri Bửu Thiên of Cần Thơ; and Bishop Joseph Nguyễn Tấn Tước, the Coadjutor Bishop of Phú Cường. Around 100,000 people attended the 40th annual Marian Days, August 3–6, 2017. Presiders included Bishop Edward Rice and Bishop Emeritus John Leibrecht of the Diocese of Springfield-Cape Girardeau; the local diocese, Archbishop George Lucas of the Archdiocese of Omaha, Bishop Kevin Vann of the Diocese of Orange in California, and Auxiliary Bishop Joseph Binzer of the Archdiocese of Cincinnati.

Other prominent presiders throughout the history of the event include: Venerable Cardinal Nguyễn Văn Thuận, Coadjutor Archbishop Emeritus of Saigon and nephew of Ngô Đình Diệm, who was detained by the communist government of Vietnam in a re-education camp for 13 years; Archbishop Emeritus Ngô Đình Thục of Huế and brother of Ngô Đình Diệm; Cardinal Raymond Leo Burke, then Archbishop of St. Louis; Cardinal Wilton Daniel Gregory, then Bishop of Belleville, Illinois and President of the United States Conference of Catholic Bishops; Archbishop Joseph Vũ Văn Thiên, then Bishop of Hải Phòng; Cardinal Daniel DiNardo of Galveston-Houston and later President of the United States Conference of Catholic Bishops; and Bishop Dominic Mai Thanh Lương, the first Vietnamese American bishop and Auxiliary Bishop of the Diocese of Orange in California.

Through 2020-2021, the Congregation had decided to give Marian Days a two-year hiatus as a concern for the health of the pilgrims of Marian Days due to the COVID-19 pandemic that was spreading throughout the United States and the whole world.

Each day of Marian Days is highlighted by a large, outdoor Mass on the CRM grounds. The Marian Days offer opportunities for Reconciliation and prayer. There is also times of benediction and workshop for young families, the youth, the Virgin Mary, Vocation, and much more. Mass is celebrated by many bishops, priests, and other religious brothers and sisters.

Pilgrims turn the surrounding area into a large campground, as many nearby residents allow pilgrims to erect tents on their lawns. The pilgrims that come earlier to set up tents are expected to stay there or risk losing their tent spots. On Wednesday, before the event begins, the brothers and priests of the congregation hold a Mass and procession in honor of Saint Joseph, who is the patron of the Marian Days. Although the celebrations are centered on liturgy, they also feature a number of other events. Communities with large Vietnamese populations set up large tents to sell traditional Vietnamese food. Proceeds go to the parishes, orphanages, or a diocese in Vietnam such as Phú Cường. Other organizations, such as a local Knights of Columbus chapter, also serve food to pilgrims in tents. Each night, performers from Thúy Nga and other groups entertain the large crowds with both folk and popular Vietnamese music.

The main schedule includes four Pontifical Masses, with two morning Masses and also many other Masses that take place outside the "Main Complex" during the day. The opening Mass on Thursday in honor of the Blessed Sacrament is opened by the local diocesan bishop (if the bishop is not present, this role is given to the director.) Following the Mass is a Eucharistic Procession around Our Lady of Peace Memorial Garden. The following day concludes with a Pontifical Mass in honor of the Vietnamese Martyr Saints, usually reserved to be presided by one of the guest bishops. On Saturday, the highlight of the event begins with the Solemn Procession of the statue of Our Lady of Fátima and the Pontifical Mass in honor of the Immaculate Heart of Mary.

At the end of the procession in honor of Our Lady of Fátima, two long firecrackers are lit, followed by the release of numerous balloons of all colors tied to two flags, one of blue and white, the Virgin Mary's colors, and the other of the South Vietnamese flag before that half of the country fell to communism. At the same time, many other balloons are released with a Rosary balloon. This tradition can be interpreted as symbolically offering the country of Vietnam to the Blessed Virgin Mary through the intercession of all gathered that day. There are also addresses written on the flags so that they could be returned. On Sunday, the closing Pontifical Mass is celebrated. Before Mass ends, the director for that year's Marian Days announces when the date of next year's event, gives a word of thanks to all those involved in the hosting of the event, and then officially closes Marian Days.

• Roman Catholicism in Vietnam

Wikimedia Commons has media related to Marian Days.

• (in Vietnamese) Chi Dòng Đồng Công Hoa Kỳ – organizer website
• (in Vietnamese) 2012 – event history
• Marian Days – Carthage Police Department