It was a daytime incident. It's just that the witnesses were high caliber AF and AEC project management staff and that's apparently why the FBI got copied on the report.
Oak Ridge map
Google Earth image with location and motion of witnesses in 18 Dec 1950 incident, plus general location and motion of UFO (the witnesses themselves were pretty vague about it). It seemed to have flown almost over the gaseous diffusion plant and was heading away to the NW.