In order to download a day for offline use you will need to follow these steps:
1) Make sure you are in a Wifi or 3G area
2) Go to your 'Downloads' screen.
3) Find the day you'd like to download
4) Click on it and a download circle should appear
5) Once it has finished downloading a tick will appear
6) Go back to your 'My Headspace' screen and find the day you've just downloaded and you should be good to go.
Please note that your downloaded sessions can not be played from the 'Downloads' screen.
7) Happy meditating!