I know that they are set to adjourn on December 18, 2015, but if they shutdown the government, can they just leave for the rest of the year? What did they do in 1995 when that happened?
The Congress can create its own rules and decide when to meet and when to be in recess. If they wanted they could decide to wait until the new year if the government were to shutdown. There is one caveat to this, however. The president does have to power to force one or both houses to convene under extraordinary circumstances, which the government being out of money would qualify.
Congress would effectively have to stay and figure out a solution if the government would shut down, but there is no explicit rule requiring them to do so. This would purely be a matter of political suicide for congress to try to avoid meeting while the government is shutdown.