Hiram Edson and certain other members of the Adventist movement were guided into the belief that the event predicted by Daniel 8:14 was not the return of Christ, but rather Christ's entrance into the Most Holy Place of the heavenly sanctuary. Adventists came to believe that the cleansing of the... More on christianforums.com

The investigative judgement is the part where God decides who is saved and who is not saved. After all, at some point, the decision has to be made, and it makes no sense to judge sinners after they've already received their sentence of death. The Seventh-day Adventist church believes that the investigative judgement began in 1844. This is on the basis of Daniel 8:14: "Unto two thousand and three hundred days; then shall the sanctuary be cleansed." In Bible prophecy, one prophetic day equals one year of real time (see Numbers 14:34; Ezekiel 4:5-6; Daniel 9:24-27). The 70 week prophecy establishes the beginning of the 2300 days, and because the last week of the prophecy involves the death of Christ, for which we have a date, we can be fairly certain about the starting date of the 2300 day prophecy, and therefore we would also know the ending date of the prophecy. But what I gather your question is that you're after "what" the investigative judgement is, rather than "when" it occurs. If you carefully examine the verse, you'll see it says that in 1844 "the sanctuary shall be cleansed." In the Torah, the sanctuary was only cleansed on one day of the year: Yom Kippur, or the Day of Atonement. You can read about the Day of Atonement in Leviticus 16. The short of it is that the sin that has built up in the sanctuary is taken out of the sanctuary and placed upon the scapegoat, and the people of Israel are cleansed of their sins. It is further described in Leviticus 23:26-32, where it says that the people who did not deny themselves on that day are cut off and destroyed. Now we know that there is a heavenly sanctuary that the earthly sanctuary was modelled after. (See Hebrews 9:23,24; Exodus 25:8,9.) We know that Christ is called our "High Priest" (see Hebrews 4:14-16). The book of Hebrews makes frequent reference to Christ in the heavenly sanctuary (for example, all of Hebrews 9). So in short, during this time period after 1844, a "day of atonement" ceremony is taking place in heaven, during which all of the sins throughout all of Earth's history are being cleansed by the blood of Christ, and being placed on the scapegoat; and the people who did not repent will be cut off and destroyed. Thus, Christ's Second Coming will end this "day of atonement" ceremony. More on reddit.com