If you're a Spotify premium subscriber, you know that you can sync individual playlists for offline listening. As of today, you can sync your entire library in one tap.
This feature became available on Android recently, and is available on iOS with the latest update. Just head to the "Songs" tab and tap the Available Offline button -- Spotify will sync up to 3333 songs, and they will be available offline for 30 days. The feature is for premium users only.