Source: http://lifehacker.com/add-google-calendar-events-directly-from-gmail-487750718
If you use email as a primary means of communication, chances are you're moving between Gmail and Google Calendar to schedule various meetings and events. Good news: now you don't have to! You can now add events directly from Gmail.
This new feature doesn't require much effort on your part. Any time someone specifies a date and/or time in an email it'll become a link that you can use to create a Google Calendar event. Not everyone has this feature just yet but can expect to see it hit their account over the next few weeks. For more information, check out the full post over at the Official Gmail Blog.
Add events to Google Calendar from Gmail | Official Gmail Blog