From http://www.hardcoregaming101.net/korea/specials/special-lost.htm#theday1:
The much hyped first episode of Mirinae's famous The Day series causes much confusion among retro gamers until today, although Mirinae's Cho Daeho clearly stated in an interview (in MyCom 8/1990, page 163) that it was never released, because they wanted to make a game that "falls nothing short of the Japanese games". The main reason for the cancellation was stated as too choppy scrolling (whether or not it ever occurred to them to just correct that certain issue rather than flushing the whole game down the toilet remains unknown). Amongst other things the screenshots show that it featured "option" upgrades, which are missing in the sequel, and a bunch of other weapons reintroduced by later games in the series.
According to Mirinae's former website, a version of the game has actually been released - it lists a board game by that title (though it is unclear whether that means an actual board game, or board as in PCB), which supposedly has been exported to Japan.