If a president fails to return a bill in 10 days without signing it, the bill becomes law. Sunday is excluded when counting the 10 days, and the 10-day period has to take place while Congress is in session.
This action happens when the president does not want to sign a bill, but he does not want to veto it either. A return veto is used when the president refuses to sign a bill and returns it to Congress to explain why the bill was not signed. With a pocket veto, the president neither signs nor vetoes the bill; if Congress adjourns while the president has the bill, it fails to become law.