When does the third trimester start?

BG
I know this has been asked before but I can't find any of the posts. I have heard both 27 weeks and 28 weeks, every time I look it up its about 50/50 with these 2 weeks so I was just curious. I'll be 27 weeks tomorrow and I just wanted to know if that starts my third trimester or if it starts next week? 

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