A.T. Robertson Commentary Luke 22:39

"And he came out, and went, as his custom was, unto the mount of Olives; and the disciples also followed him." — Luke 22:39 (ASV)

As his custom was (κατα το εθος). According to the custom (of him). It was because Judas knew the habit of Jesus of going to Gethsemane at night that he undertook to betray him without waiting for the crowd to go home after the feast.