Sony explained the offer in a statement on their official blog:
This is a special demo and not available in the PlayStation Store.
The trial will automatically be downloaded to your PS3 one night before the launch of the game. The trial however, won’t be playable until it’s official launch which is the 21st.
Note that you’ll have to enable ‘Automatic Updates’ on your PS3.
XMB > Settings > System Settings > Automatic Updates
The said date and time is based on the American time-zone. Hence, if you’re based in Europe then you’ll have to do some calculations to find the correct time.
For now anyone who has pre-purchased Counter-Strike: Global Offensive for PC on Steam can play it right now.