The 5-year ACS files were released by Geolytics for the first time in 2009 because that was when the data became available down to the block group and tract level.
The 5-Year ACS files are the replacement for the Long Form – giving us detailed information about the American population at small geographies, like Block Groups and Tracts
The ACS samples nearly 3 million addresses each year, resulting in nearly 2 million final interviews. The annual ACS sample is smaller than that of the Census 2000 Long Form sample, which included about 18 million housing units. As a result, the ACS needs to combine population or housing data from multiple years to produce reliable numbers for small counties, neighborhoods, and other local areas
Every year the US Census Bureau (USCB) will release a new single year ACS survey and replace the oldest year with this new data for the 1- and 5-year estimated numbers.