"Historically, the most active for ghost phenomena are between 9 PM and 6 AM" says Tom Ogden in The Complete Idiot's Guide to Ghosts and Hauntings. But, he notes that spirit behavior can happen at any time.
In The Paranormal Investigator's Handbook, editors Hope & Townsend recommend ghost vigils at night. This is not because ghosts are necessarily more active then, but because it's easier to observe unusual phenomena at night. You'll have fewer distractions.
Witch Patricia Telesco observes in Ghosts, Spirits and Hauntings, "Ghosts are more readily seen and contacted at midnight (the time in between day and night), and on the anniversary of their death."
We've encountered ghosts during the daytime as well as after dark. We prefer the hours around dusk through early evening, but in some cases - such as our investigation of The Myrtles Plantation - activity increased dramatically after 8:30 PM
There's less solar interference after sunset. Just as distant radio stations are easier to pick up at night, ghosts can create more dramatic effects with less energy after the sun goes down.
We also believe in the popular idea that between times are excellent for ghost research. Perhaps a door opens between the worlds when conditions are slightly unstable.
The between times include dusk and dawn, and both the Equinoxes and the Solstices, when the seasons change. Since Halloween (Samhain) is the traditional end of the agricultural year, many more hauntings occur on that night. We've had good luck at the opposite end of the agricultural calendar, at the last day of April.
And, we've heard increased reports around the end of the calendar year, at the end of December and start of January.
Some people believe that it's easiest to contact the other side while the clock is chiming midnight, another between time.
When you're planning an investigation, find out when others have experienced anything odd at that location. We've heard few stories from normal morning hours, or even early afternoon. Generally, manifestations occur after 3 PM, and most are after dark.
Keep a diary of your encounters with ghosts. See if there is any pattern to hauntings that you witness. If so, it may be a characteristic of the ghosts. Or, you may have heightened sensitivity to the paranormal during certain times.
Compare this pattern with planetary hours - mentioned above - and your biorhythms, too. Every factor, no matter how unlikely, must be considered.
No matter what rules or patterns you establish for your research, there will always be exceptions. However, ghost hunting is still trial-and-error for most of us. No one can say with certainty that one hour is always better than another for your research.
Keep testing and comparing results with others, even if you think that you've found a consistent pattern. We're far from knowing what causes hauntings, and your information could be part of a breakthrough discovery that will help all ghost researchers.