When does 3rd trimester start?

Tracey β€’ Dog mama 🐢 dating the most amazing man πŸ’ fitness lover πŸ‹πŸ½β€β™€οΈ expecting a baby girl January 11th πŸ‘ΆπŸ½πŸŽ€

What does everyone consider the start of the 3rd trimester?? Is it 26 weeks or 28?