I read in grade school (early 70s), but I can’t remember the name of it.
Little girl had just moved to house on outskirts of small town New England coast of US. She likes to play in fog. One day meets another little girl. She speaks oddly, old-fashioned. Is dressed old too. But they have good time berry picking. Go to strange girl’s house. Also very old fashioned, more like a cabin. But girl thinks kinda cool. 
The odd girl’s mom fixes a couple of berry pies. Wraps one up for her to take to her mom. When she gets home, fog is lifting and pie is gone. The two girls meet often, always in fog. They become fast friends. 
Finally mom wants to meet this family. Daughter takes her to spot she KNOWS the village is standing. Nothing there. Just bright sunshine on the empty field. Few blocks away is a replica of a Puritan village but isn’t the same. She keeps saying things were in the wrong place. Never saw little girl again, foggy or not. 

Wish I could remember the name but was in like 4th maybe 5th grade at the most.”