Mr. Herbert W. Armstrong explains this thoroughly in his booklet:
Title: Which Day is the Christian Sabbath?
Author: Herbert W Armstrong
Date: 1962, 1964, 1968, 1970, 1971, 1972, 1976
Type: Booklet
Type Media: PDF
https://cog-hwa.org/pub/bb/sabbath.pdf
Other material produced by or under the direction of Mr. Armstrong:
Title: Which Day is the Sabbath of the New Testament?
Author: Herbert W Armstrong
Date: 1952, 1971
Type: Booklet
Type Media: PDF
https://cog-hwa.org/pub/bb/testament.pdf
Title: Mystery of the Ages
Author: Herbert W Armstrong
Date Published: 1985
Type: Book
Type Media: PDF
https://cog-hwa.org/pub/bb/mystery.pdf
Title: The Incredible Human Potential
Author: Herbert W Armstrong
Date: 1978
Type: Book
Type Media: HTML
https://cog-hwa.org/pub/bb/potential.pdf
Title: The United States and Britain in Prophecy
Author: Herbert W Armstrong
Date Published: 1980
Type: Book
Type Media: PDF
https://cog-hwa.org/pub/bb/britain.pdf
Title: The Wonderful World Tomorrow -- What It Will Be Like
Author: Herbert W Armstrong
Date: 1966, 1973, 1979
Type: Book
Type Media: PDF
https://cog-hwa.org/pub/bb/tomorrow.pdf
Title: Pagan Holidays -- or God's Holy Days -- Which?
Author: Herbert W Armstrong
Date: 1976
Type: Booklet
Type Media: PDF
https://cog-hwa.org/pub/bb/holydays.pdf
Title: The Plain Truth About Easter
Author: Herbert W Armstrong
Date: 1957, 1973
Type: Booklet
Type Media: PDF
https://cog-hwa.org/pub/bb/easter.pdf
Title: The Resurrection Was Not on Sunday
Author: Herbert W Armstrong
Date: 1952, 1971, 1972
Type: Booklet
Type Media: PDF
https://cog-hwa.org/pub/bb/resurrection.pdf
Title: The Plain Truth About Christmas
Author: Herbert W Armstrong
Date: 1952, 1972, 1974
Type: Booklet
Type Media: PDF
https://cog-hwa.org/pub/bb/xmas.pdf
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