On Friday, the D.C. Circuit Court of Appeals ruled that the constitutional authority to fill a vacancy can be used in the time between the end of one Congress and the start of another, or when there is a formal break at the end of one session. In other words, it cannot occur during any other mid-session break. As such, three appointments made by President Obama to the National Labor Relations Board were found unconstitutional.
With this ruling, lower courts are now split on this issue of separation of powers, and it’s likely that the issue could go before the Supreme Court. See SCOTUSBlog’s full analysis here.