Apple Adds A Download Later Feature to iTunes
Apple has recently added a new option for downloading content for iTunes purchases. This feature will be for both the Mac running iTunes 11 or newer and iOS devices running iOS 6 or later.
The way it works is a message will come up asking you if you would like to download that item now or later (shown above). This will come up when downloading large files like TV shows or movies. If you choose to download it later, iTunes will store it in your purchases list to be downloaded at a later date.
If you need support with this feature you can refer to this support article by Apple.