Freya,
namaste,
Ghosts are not always easy to get rid of. I know some ghost-busters who have a difficult time. :o}
I know some things that will move them on, if you would like to email me. Probably very similar to what Firekeeper tells you. (hi Peter!)
My feelings about ghosts, are that if they are not disruptive, and only there once a month, leave them alone.
I would suspect that there is a source of water near the house. If not above ground, then underground and quite near. Spirits tend to be more visible near moving water.
It may also be a spirit that is attached to one of them instead of the house, and their energy on the 16th makes him more visible.
Also what is going on around the 16th at that house? Why only the 16th, in other words. Is he visiting the other people in the neighborhood the rest of the month? Sort of like your maiden aunt who lives with all the relatives a short time every year?
How do they know the ghost is friendly? Did the manifestations begin when they first moved in, or take time to develop?
Blessings
Priestess