In the early 1800s, a Baptist farmer-preacher named William Miller embarked on an intensive study of Scripture. His methodical, verse-by-verse approach led him to Daniel 8:14, which states: "Unto 2,300 days, then shall the sanctuary be cleansed." Using the Day-for-a-year principle, Miller calculated that this prophecy would conclude in 1843.

However, Miller's interpretation was based upon the idea that the sanctuary represented the earth. Therefore, if the sanctuary represented the earth, then its cleansing could only occur when Christ returned to destroy the wicked and purify the earth. As a result, Miller interpreted the 2300-day prophecy to mean one thing—that the Second Coming of Christ was near. He declared, "Jesus Christ will come again to this earth, cleanse, purify, and take possession of the same with all the saints." - Memoirs of William Miller p. 180 

Two years after Miller began publicly preaching this message, a spectacular event occurred. The Great Meteoric Shower of 1833 was seen as a fulfillment of Matthew 24:29 when Jesus said, "the stars shall fall from heaven." This event gave credibility to the imminent return of Christ, and many were converted to the message. Thus, the Advent Movement was born.

                                                          In mid-August of 1844, the Advent people were gathered at a camp meeting in Exeter, New Hampshire. A large focus of this gathering was with the hope that God would give them clarity regarding the 2300-day prophecy. As they reexamined their positions, a man by the name of Samuel Sheffield Snow proposed a crucial correction to the date of the prophecy that they hadn't considered. Rather than using the spring equinox date of March 21 to determine the timing of the prophecy, Snow argued they should align their calculations with the Jewish calendar and its reckoning of the Day of Atonement, which fell on the 10th day of the seventh month. His recalculations pointed to October 22, 1844, as the true date for the sanctuary's cleansing.

The Advent believers saw the truth of Snow's revelation. Even William Miller examined this position and eventually accepted the new date. Thus, the Millerite followers became reinvigorated with this new reckoning of the 2300 days and this small delay in the fulfillment of this prophecy became known as the "seventh-month movement." 

However, October 22, 1844, came and went like its predecessor and the Advent Movement received a devastating blow. Many of the believers abandoned the movement. Some even joined the ranks of those who mocked the Advent Movement. Others even abandoned the faith altogether. This bitter experience resulted in what history remembers as the "Great Disappointment."

                                              October 23, 1844, the day after the Great Disappointment, proved pivotal for the small remnant of the Advent Movement who refused to abandon their faith. As they continued to plead and agonize with the Lord for an understanding of why the prophecy didn't happen as expected, a fellow Advent believer named Hiram Edson reported receiving a vision while walking through a cornfield. Little did he know that this vision would totally reshape the Advent Movement's understanding of the cleansing of the sanctuary.

According to Edson, Christ had not come to the earth on October 22 because He had instead moved from the Holy Place to the Most Holy Place in the Heavenly Sanctuary. This new revelation suggested that the cleansing of the sanctuary referred to in Daniel's prophecy was not in reference to the earth, but of heaven.

This revelation propelled the small group to study the concept of a Heavenly Sanctuary in earnest. They discovered that Scripture indeed spoke of a sanctuary in heaven. Revelation 11:19 confirms this when it says, "And the temple of God was opened in heaven, and there was seen in his temple the ark of his testament: and there were lightnings, and voices, and thunderings, and an earthquake, and great hail." In this manner, the Advent believers began to understand that the earthly Jewish tabernacle was simply a model of the Heavenly Sanctuary. As a result, the early Adventists would unlock new prophetic truths that had not before been understood, and a new phase of the Advent Movement was born.

                                                    During this period of theological development, God raised up Ellen White as a prophetic voice to guide the emerging new version of the Advent Movement. Under inspiration, she later confirmed the centrality of this doctrine, stating that Daniel 8:14 was "the foundation and central pillar of the Advent faith."

However, the 2300-day prophecy wasn't the only truth Ellen White would confirm. After reading a tract by Joseph Bates regarding the Sabbath, Ellen White and the Advent believers also embraced the Bible's original day of the rest. 

In 1863, these Sabbath-keeping Adventists officially organized as the Seventh-day Adventist Church—a name reflecting both their belief in Christ's Second Advent and their commitment to God's Sabbath commandment.