It's fairly commonplace in the federal government to have 'use it or lose it' leave. Only a certain amount of vacation hours can roll over from one year to the next, so those which you don't use and don't roll over are lost.

While it's not mandatory to take that leave, it's a very good excuse for why you want to take several days off.

See more here: http://www.opm.gov/policy-data-oversight/pay-leave/leave-adm...