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I rather wish I'd reviewed this film side by side with Larry Buchanan's remake, IN THE YEAR 2889. In that review I complained a little about the acting "histrionics" of DAY, in contrast to the "somnambulistic" performances dominating the later film. Yet now I'd say that the acting in DAY isn't all that bad; it's just that the characters are one-dimensional types, giving the actors little to work with.
In contrast to THE SHE CREATURE, written by DAY's scripter Lou Rusoff the next year, the characters of this post-nuclear drama are no more than schematic figures. In the aftermath of nuclear conflict, an older man named Maddison flees with his grown daughter Louise to a box canyon out west. Lead permeates the canyon-walls, in theory shielding the occupants from fallout, which takes the form of radioactive vapors that swirl outside the canyon yet somehow can't pass the canyon's walls. Maddison is a rather circumscribed Noah, who hopes to repopulate the polluted world with a marriage between his daughter and her fiancee. However, the fiancee is lost in the chaos, thus making Louise "up for grabs" when other survivors of the conflict find their way into the canyon. These include stalwart scientist Rick, nasty gun-wielding hood Tony, Tony's aging moll Ruby, an old guy with a burro, and a fellow who's contracted radiation poisoning. In addition, Maddison has a fear of precipitation that Noah could not have imagined, since the next big rain may be radioactive-- and thus will seal the fate of the last humans.
These repeated apocalyptic references-- even presenting mankind's devastation as part of God's plan in the opening prologue-- are the strongest symbolic aspect of DAY, an aspect pretty much mucked up in the late YEAR 2889. Significantly, Maddison doesn't gather any animals into his redoubt, not counting the old prospector's burro. He relates, though, that he's seen how radiation mutated test animals under military experiments, so it's understandable that he's a little reluctant to bring other creatures under his aegis. The proper breeding of humanity is Maddison's main concern, and thus there's a continuing battle between "good guy" Rick and "bad guy" Tony to see who will get access to the fertile female.
Further, mutation is an ongoing concern, for the man with radiation poisoning begins to develop strange habits, making it seem like he may be mutating to tolerate the fallout. There's also a humanoid monster stalking the area, and though it only eats contaminated animals, it seems able to communicate with Louise on a psychic level. She even claims that the creature calls her by name, opening up the possibility-- never confirmed in the script as filmed-- that the monster may Louise's lost fiancee, rapidly mutated by the fallout. This deformed suitor, who in his absence has been eclipsed in Louise's eyes by the square-jawed Rick, is something of a loose parallel to Ruby, who is thrown over by Tony when he starts obsessing over Louise. Rusoff shows considerable empathy for Ruby, a former exotic dancer, just as the scripter did for the camp-follower from SHE CREATURE.
In contrast to Richard Matheson's post-apocalyptic novel I AM LEGEND-- published the year before DAY THE WORLD ENDED-- Rusoff's characters view mutants as a stain upon God's creation. Thus it's no coincidence that when nature's rain at last comes to the canyon, it's a rain representing the mercy of God, a deluge that dissolves the humanoid mutant and may also function to get rid of any others skulking around the decimated planet. Thus humankind gets another chance at survival, and though everyone in the group dies except for Rick and Louise, there's also a last-minute revelation that other normal humans have survived the cataclysm.
This was the fourth directorial credit for Roger Corman, though his work on THE BEAST WITH A MILLION EYES was not credited.