Shutdown of the Federal Government: Causes, Processes, and Effects [November 30, 2017] [open pdf - 1MB]
"When federal agencies and programs lack funding after the expiration of full-year or interim appropriations, the agencies and programs experience a funding gap. If funding does not resume in time to continue government operations, then, under the Antideficiency Act, an agency must cease operations, except in certain situations when law authorizes continued activity. The criteria that flow from the Antideficiency Act for determining which activities are affected are complex. Failure of the President and Congress to reach agreement on full-year or interim funding measures occasionally has caused shutdowns of affected federal government activities. The longest such shutdown lasted 21 full days during FY1996, from December 16, 1995, to January 6, 1996. More recently, a funding gap commenced on October 1, 2013, the first day of FY2014, after funding for the previous fiscal year expired. Because funding did not resume on October 1, affected agencies began to cease operations and furlough personnel that day. A 16-full-day shutdown ensued, the first to occur in over 17 years."
CRS Report for Congress, RL34680
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html