The event attachment Lab is now fully launched in Calendar by default. The event attachment feature allows users to attach meeting notes, presentations, or other Drive files to the calendar event. Once the files are attached to the calendar event, they become available for all event attendees. The Add attachment link appears just under the 'Description' in the event.
As an added bonus, Drive also confirms that all event invitees have access to the attached file. If invitees do not have access to attached files, users receive a warning message with the ability to grant access before saving the event and sending out to attendees. In addition, developers can now add Drive files to Calendar events created through the Calendar API.
For more information, see Event attachment improvements for Google Calendar web and API.