Seasonal housing units are those intended for occupancy only during certain seasons of the year and are found primarily in resort areas. Housing units held for occupancy by migratory labor employed in farm work during the crop season are tabulated as seasonal. As of the first quarter 1986 vacant seasonal mobile homes are being counted as a part of the seasonal housing inventory.
Source: US. Bureau of the Census
Release: Housing Vacancies and Homeownership
US. Bureau of the Census, Housing Inventory Estimate: Seasonal Housing Units in the Midwest Census Region [ESEASONMWQ176N], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/ESEASONMWQ176N/, September 1, 2015.