See the chart in the link below to show agencies impacted.
With the threat of a government shutdown looming this week, we know that only about 25 percent of the federal government will be impacted, but which part is that? See this Bloomberg story, "Here’s What Would Happen If There’s a Government Shutdown in December."
It should be noted that in the story it states, "At the Department of Homeland Security, the overwhelming majority of border patrol, emergency management and immigration enforcement staff would be able to keep doing their jobs, though with their pay delayed."
Sorry FEMA employees, no extended holiday for nonessential personnel. I don't know the exact status, but it could be that "all FEMA Employees" are designated essential. That is how I'd do it if I were the FEMA administrator.
Claire Rubin shared the link above.