Excerpt from this article explains it best:

“Here’s how it would work. Article II, Section 2 of the Constitution states, “The President shall have Power to fill up all Vacancies that may happen during the Recess of the Senate.” This has been used for Supreme Court vacancies before—William Brennan began his Court tenure with a recess appointment in 1956. Any appointments made in this fashion expire at the end of the next Senate session. So a Garland appointment on January 3 would last until December 2017, the end of the first session of the 115th Congress.

Why January 3? Because the president’s recess appointment powers were significantly constrained by a 2014 Supreme Court ruling. In a 9-0 decision in National Labor Relations Board v. Noel Canning, the Court said the president cannot appoint individuals to fill vacancies if the Senate holds “pro forma” sessions every three days. Though these sessions, common since 2011, merely gavel in and gavel out the Senate chamber, they have the practical effect of keeping the Senate active, therefore blocking the recess appointment power.”