The American Community Survey (ACS) is an annual survey held by the U.S. Census Bureau. The ACS supplements the Decennial Censuses by providing these annual updates and in 2010 replaces the Census Long Form.
The ACS annually receives data from approximately 2.5% of the U.S. population. This amounts to 12.5% for the 5-year estimates. (Census 2000 Long Form sampled 17% of the population.) This increases the sampling error of the ACS to 1.3 times larger than the 2000 Long Form. However, arguments in favor of the ACS are (1) annual release of statistics, and (2) the maintaining of a permanent and professional staff (as opposed to temporary employees) may result in lower rates of non-sample error.
Regarding the counting of the U.S. population to determine the number of representatives in the House of Representatives, Article I Section 2 of the U.S. Constitution states that "The actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they shall by Law direct."
The first Decennial Census was counted in 1790 and has been counted and reported every 10 years since then.