William Miller had not set a specific day for the Advent, but expected it at some time during the "Jewish year 1843," that is, the year 1843/1844 from spring to spring. This new definite date, which Miller did not preach and did not accept until shortly before it came, was calculated by several of his colleagues.
The year 1844, instead of 1843, was arrived at by Apollos Hale, Sylvester Bliss, and others through the correction of a one-year error in computation from B.C. to A.D. dates. The month and day, worked out chiefly by Samuel Snow, were selected because (1) the expectation of the Advent was based chiefly on the calculation of the 2300 days (counted as years), according to the prophecy, "Unto two thousand and three hundred days; then shall the sanctuary be cleansed" (Dan. 8:14); (2) the annual ritual cleansing of the ancient Hebrew sanctuary took place on the tenth day of the seventh month, called the Day of Atonement (see Lev. 16:16-19, 29-34); and (3) this Jewish calendar date was computedâ€”not according to the current Jewish calendar, but according to an older form attributed to the Karaite Jewsâ€”as the equivalent of Oct. 22 in 1844.
This interpretation was developed principally by Snow out of Miller's suggestion (letter of May 3, 1843) that just as the ancient Hebrew spring festivals (Passover, Pentecost) were types of the death and resurrection of Christ, so the autumn festivals (Day of Atonement, Feast of Tabernacles) typified the Second Advent.
Miller had mentioned several events occurring on the tenth day of the seventh Jewish month (the Day of Atonement), such as the cleansing of the sanctuary, its furnishings, and its worshipers; the sounding of the jubilee trumpet signalling the release of all Israelites in bondage, a type of the final redemption; and the atonement made on that day, followed by the coming of the high priest out of the Holy of Holies, typical of Christ's priestly ministry ending at His second coming. Thus many looked to the autumn of 1843 "with much interest." Then, as Himes relates: "Snow fully embraced the opinion that, according to the types, the advent of the Lord, when it does occur, must occur on the tenth day of the 7th month; but he was not positive as to the year. He afterwards saw that the prophetic periods do not actually expire until the present 1844; he then planted himself on the ground that about the 22nd of Octoberâ€”the tenth day of the seventh month of this present yearâ€”must witness the advent" (Advent Herald 8:93, Oct. 30, 1844).
The autumn expectation was based on the idea that the 70 weeks of years (beginning synchronously with the 2300 years) began and ended in the seventh month; and on the application to Christ of the types of the ancient Mosaic festivals. The date was based on the following reasoning: Since Christ, our Passover, was crucified on the fourteenth of the first Jewish month, the day prescribed for the slaying of the Passover lamb, and because He rose again on the day of the wave sheaf (the sixteenth of the same month), it was logical to expect that Christ our great High Priest would fulfil the antitype of the Day of Atonement by coming from the Holy of Holies, or heaven, on the tenth day of the seventh month to bless His waiting people and to announce the beginning of the year of jubileeâ€”the millennium.
It was in February 1844 that both Hale and Snow published their revised reckoning, ending the 2300 years in 1844, and soon afterward Snow fixed on the tenth day of the seventh month, 1844. But acceptance was slow. Not until after midsummer, when Snow began to preach on the subject, notably at the camp meeting at Exeter, New Hampshire, in August, did the movement catch fire.
In addition to the Day of Atonement type, Snow used the parable of the ten virgins in a new way as evidence for his dating. The Millerites had expected the Second Advent at least by the spring of 1844. Between that first expectation (in the spring of 1844) and the second (in the autumn) there were six monthsâ€”half a year, or half a prophetic day. This, said Snow, was the "night" of waiting, when the Bridegroom delayed His coming; and at midsummer, the midway point of this interval, corresponding to midnight, came the seventh-month message, representing the cry, "Behold, the bridegroom cometh; go ye out to meet him." The Millerites had called their message the midnight cry, but this new message was called by its adherents the true midnight cry, to which the other had been preliminary.
As the date approached, enthusiasm mounted, though not all the Millerites joined the seventh-month movement. One by one the Millerite leaders, who had been the last to take part in it, accepted the seventh-month message. William Miller and J. V. Himes, his lieutenant, came to the conclusion early in October that the movement must be the Lord's doing, and they too looked for the Advent on that October day.
Just as the great surge of enthusiasm over the October date separated the Millerites most completely from the world at large, so after the Great Disappointment, when that day passed, it was the question of the significance of this seventh-month movement, the "true midnight cry," that drew the sharpest line of cleavage between the Millerites themselves. Had it been a colossal blunder, or had it been truly a fulfilment of prophecy-though not the fulfilment they had expected-and had God indeed been leading them in it, testing their devotion and their readiness to meet Christ?
In the aftermath the majority, including most of the leaders, came within a few months to the conclusion that it was "not a fulfilment of prophecy in any sense," that their prophetic chronology had been wrong, and that the fulfilments were yet in the future. Those who held that the movement had been led of God concluded that the timing was right and sought other explanations of their disappointment.
From the latter came the little groups that later became the Seventh-day Adventists. These refused to "deny their past experience," as most of the others seemed to them to have done. They sought another meaning in it and arrived at the conclusion that the cleansing of the sanctuary was not the return of Christ, but involved another phase of His priestly ministry before His return to this earth (see Sanctuary) -- Seventh-day Adventist Encyclopaedia.