How to explain this?

The cynic would say it just reflects the source of the reports. Most reports in the 1950's and 1960's were from local newspapers and the US Air Force Blue Book project. Air Force staff work Monday to Friday; on the weekends there is no one there to take an initial phone call of a report; the callers may have given up immediately. Likewise, local newspapers had only skeleton staffs working on weekends. In the late 1990's and 2000's, most UFO sightings were self-reported on the Internet. This shifted the primary days to the weekends, when more people were about in the outdoors and had the chance for a sighting.

Furthermore, over the decades, the world became increasingly urbanized. There were fewer farmers, ranchers, and workers out and about on weekdays.

On the other hand, this does not necessarily explain the steady progression through the week, or the fact that sightings in the 1940's were predominately on Sunday. The movement of one day over a 10 year period would indicate a gain in some kind of UFO daily time cycle 1/3,652.5 = 0.0273785% = 23.65 seconds greater than the length of an earth day.... which does not evidently correspond to any known celestial or earthly time variation.

Any suggestions?