What Time of Year Are Mice Most Likely to Enter Houses?

In Manchester, the seasonal transition tracks the first fall frost closely. Using NOAA's 1991–2020 Climate Normals for the Manchester station, there is about a 50% chance of a first 32°F frost by around October 19, rising to roughly 80% by about October 29. Frost risk begins building in early October and is near-certain by month's end. Those dates matter because the cooling nights that precede and accompany frost are exactly the trigger that drives mice indoors — not a fixed calendar date, but a temperature threshold that Manchester reliably crosses in that window.

Field guidance for the Northeast puts peak indoor mouse activity from late August through November, sharpening in the weeks bracketing the frost dates. Scouting activity can begin as early as late August as outdoor food starts to thin, building to the sharpest entry pressure through September and October, then continuing into November as hard frosts become routine and snow arrives.

While house mice can technically survive at 24°F with food and nesting material available, they strongly prefer indoor harborage — and once inside a heated home they breed continuously, so a single mouse that enters in early October can become an established colony by mid-winter. That's why the actionable New Hampshire recommendation is to complete exclusion and sanitation before the frost window, ideally by late September to early October, rather than reacting once scratching starts in the walls.