A new level will become available daily. All levels that are unlocked while the game is installed can be accessed at any time. However there may be levels that became available before you started playing. If this is the case, they can be unlocked by watching a video advertisement. To replay a level, tap the "Settings" icon then tap a day on the calendar. If you have to unlock that level beforehand, a lock will appear on that date. If you already unlocked it, you can tap the day to play it.