Looking at the best 241 cases of interest from 2017 from the Mutual UFO Network (MUFON), the timeline is narrowed where we see 45.6 percent of cases occurred local time between 8 p.m. and midnight, according to data collected from UFO Cases of Interest: 2018 Edition.
The cases were fairly evenly spaced between those four hours. 8-9 p.m., 27 cases; 9-10 p.m., 32 cases;10-11 p.m., 31 cases; 11 p.m. - midnight, 20 cases.
While UFOs were reported in every one-hour span around the clock, the largest gathering of cases was reported during those four hours. UFO Cases of Interest: 2018 Edition is available at Amazon.com.