Google is rolling out a new interface for Google Calendar.
Google Calendar is currently set to default to the classic calendar, with the new calendar an option. If you would like to check out the new calendar, there is a blue button rolling out to users that will allow you to Use new Calendar.
Clicking on the Use new Calendar button will give you the new Google Calendar interface. To revert back to the old interface, click the gear icon in the upper right and select Back to classic Calendar.
The classic calendar will be available until February, at which time the new Google Calendar interface will become the default and the classic calendar will no longer be available.
For more help with the new Google Calendar, check out the help page from Google.