When does the third trimester start?

Stacie • Mother to Ben 👦🏽, Bash 👦🏽, and Braxton 👶🏽, 420 enthusiast 🔥💨, lover of life 😍🤘🏾
I'm 28 weeks exactly today! Am I considered to still be in the second trimester or have I finally made it to the third trimester?