The following groups are categorised as Christians.

<<For statistical purposes: Groups which self-identify as part of Christianity include (but are not limited to): African Independent Churches (AICs), the Aglipayan Church, Amish, Anglicans, Armenian Apostolic, Assemblies of God; Baptists, Calvary Chapel, Catholics, Christadelphians, Christian Science, the Community of Christ, the Church of Jesus Christ of Latter-day Saints ("Mormons"), Coptic Christians, Eastern Orthodox churches, Ethiopian Orthodox, Evangelicals, Iglesia ni Cristo, Jehovah's Witnesses, the Local Church, Lutherans, Methodists, Monophysites, Nestorians, the New Apostolic Church, Pentecostals, Plymouth Brethren, Presbyterians, the Salvation Army, Seventh-Day Adventists, Shakers, Stone-Campbell churches (Disciples of Christ; Churches of Christ; the "Christian Church and Churches of Christ"; the International Church of Christ); Uniate churches, United Church of Christ/Congregationalists, the Unity Church, Universal Church of the Kingdom of God, Vineyard churches and others. These groups exhibit varying degrees of similarity, cooporation, communion, etc. with other groups...>>

http://www.adherents.com/Religions_By_Adherents.html

Most of these religions I know very little about but I do know that Anglicans, Roman Catholics, Uniting (Methodist/Presbyterian), Lutherans all celebrate a regular church gathering/mass/eucharist on Sundays.

However, many other Christian religions celebrate their gathering on Saturdays.

Why is this?

I could cut and paste paragraph after paragraph from various sites but this one seems to cover more than the others I`ve seen.