With offline calendars, you can access your calendar offline to view events from the last 4 weeks, or any time in the future. Note that you cannot create or edit events while you are offline, and you can't email guests in any event.
To set your calendar to be available offline:
Click on the Settings cog near the top-right corner of the screen
Choose Settings
Click on the Offline option on the left-hand side of the screen
Check the box to Turn on offline calendar
You'll be prompted to reload your calendar - click on Reload now
The sync button shows that your calendar will be available offline: