First-day Adventists believe(d) that Christ's coming was going to occur very soon. They are one of the primary groups that grew out of William Miller's preaching toward the middle of the 19th century. As their name states, they observed the first day of the week, Sunday.
Seventh-day Adventists are one of the descendants of the previous group and most distinctly worship on the seventh day of the week, Saturday. There are some other distinctions which we cannot go into detail at this time.
(Eighth-day) Adventist Futurists are a conceptual society. They believe that not only is the seventh day the day of rest, but additionally, they observe the hours after sunset on Saturday night and ending with sunrise on Sunday morning, as well. They focus on the spiritual beauty of the Sabbath and not on the strict letter-of-the-law. (This does not necessarily mean that 7th-Day Adventists do not also focus on the spiritual beauty of the Sabbath.)
(Eighth-day) Adventist Futurists have as one their primary goals to escape planet Earth's eventual human-engineered destruction by escaping to habitable worlds within this solar system and, eventually, to worlds surrounding nearby stars. Progressive (Eighth-day) Adventist Futurists go one step further and observe any 24-hour period commencing on any day of the week at sunset and concluding at sunset on the following day because this will be the norm when the weekly cycle will cease to be meaningful. The sun never sets in outer space.
Perfect Futurists are theorized as being the descendants of the (Eighth-day) Adventist Futurists when they have successfully engineered Macro Life worlds which can exist in-between worlds and are said to be the eventual and preferred living environments for space-faring humans. [Scientist Dandridge M. Cole originated the term "Macro Life" in his 1961 book The Ultimate Human Society.]*
Please see Adventism's New Horizons for more in-depth treatment of this future society.
* "Cole conceived Macrolife as a possible next step in evolution, potentially as momentous as the transition from single-celled to multicelled life. Units of Macrolife, self-contained human societies in planetoid colonies or elsewhere, would have the capacity for growth, motion, reproduction, self-repair, and response to external stimuli. He developed further details in his 1961 The Ultimate Human Society and in subsequent books."