When does 3rd trimester start

Mandy • 💏2012 🎀👶🏼1/10/14👼🏻+👼🏻+👼🏻+👼🏻2019👼🏻6/30/19,👼🏻5/31/20

Hey ladies is it 27 weeks or 28 weeks that’s the start of the 3rd trimester?