If you live adjacent to a forest, they tend to look for a warm place once the nights start getting colder, around early October. We have 500+ acres of public use land adjacent to our back yard in souther NH. I walked into my four-year-old son's bedroom last night to find over 200 of them on the walls and ceiling. He thought it was cool! Once I'd cleaned them out, I started in on the dining room...the hallway...the family room... The migration generally ends by early November.