At what week are you officially in 3rd trimester?

Danielle • 33yo, first time mom and so excited for it all!! Married 10.03.2014 and we are over the moon about our Nov baby boy!
Hi ladies!!!!
I've heard so many different answers to this question and read conflicting info as well...
When are you OFFICIALLY considered to be in your third trimester?
We will be 27 weeks tomorrow.
Hope you are all doing well and baby bumps are happy. 😀