The 2018 shutdown began at midnight eastern time on January 20, 2018. Senate Democrats voted against funding the federal government because it did not include DACA (Deferred Action fro Childhood Arrivals) (DACA) legislation.
[See the Research Connection: DACA in the News Tutt Library blog post from 9/4 for more information about DACA.]
Since 1976, there have been 19 separate government shutdowns or "spending gaps". Previously, the last shutdown was in 2013. It lasted 17 days from October 1–16. The deadlock centered on "Obamacare" as structured in the Continuing Appropriations Resolution for 2014. [Appropriations for Fiscal Year 2014]
A government shutdown occurs when Congress and the President fail to pass legislation to fund government operations. It is the responsibility of the legislative branch to appropriate government funds according to the Antideficincy Act. That act requires that the federal government begin a activities such as furloughing non-essential personnel and curtailing agency activities and services. Most federal agencies continue to operate during a shutdown. They minimize all nonessential operations and obligations in the belief that no one wants to stop government activity for long and that legislation will be passed quickly.
To read the debates in Congress, use Fedsys to see the recent Congress Record. It contains a record of everything said in both Congressional houses.