The first book is about a modern day boy who lives in New England. Every time the fog rolls in, a mysterious village of Revolutionary War era appears in the woods. The boy develops a relationship with the villagers and eventually realizes that his own father also had a similar experience. Either the boy or his father was gifted a pocketknife by the ghost villagers.
You might try ‘Fog Magic’ by Julia Sauer. There are a few differences, but there are also quite a few similarities, including the village appearing only during the fog, the ability to see/enter the village being something that the main character shared with their parent/grandparent, friendship with the past villagers, and a gift from the villagers. The differences are that the main character is a girl rather than a boy, the village is in Nova Scotia instead of New England, and at the end, the villagers give her a kitten instead of a pocket knife.
“The fantasy story centers on eleven-year-old Greta Addington. One child in every generation of Addingtons is able to experience the special magic of Blue Cove, Nova Scotia. In fair weather, ruined buildings are all Greta sees, but when the fog rolls in she can travel back in time to visit the village and its inhabitants. While there she has a friend to play with, and the people refer to her as coming “from over the mountain”. Greta is especially eager to go there on her twelfth birthday, but she has to wait till night for it to become foggy. That night in Blue Cove her friends give her a kitten, and Greta leaves realizing she will never be able to return.”
That’s the story only I’m sure was a little boy and was new england. As I recall, the boy received a pocket knife or his father did and the boy recognized it somehow.
Still not New England, but I did stumble across another book that sounds sort of similar: ‘In the Circle of Time’ by Margaret Jean Anderson (1979).
“A time-shift fantasy is set in Scotland, where Robert Guthrie meets a new classmate from America, Jennifer, as each explores the ancient circle of stones. It is Jennifer who first sees, in the grey mist, a group of strange, dark-haired people, but it is not until a later visit that the two move into another time and become involved in the struggle of those people, the peaceful members of a cooperative society of the future who are struggling against the domination of the Barbaric Ones. Most of the story has to do with Jennifer’s and Robert’s adventures in their time-travel, and the complex structure of these may limit some readers’ enjoyment of the book; there is little depth in characterization, although the characters are adequately drawn, and little meshing of realism and fantasy, save for the end of the story, when the two time-travelers, listening to Robert’s grandfather tell them of a similar experience, realize that one of the people the man had seen was his own grandson.”
I can’t lay my hand on Fog Magic this instant, but in the end, Greta’s father does show her the pocket knife he was given in the village.