When does the first trimester end?

Week 13. A preg is 40 completed weeks from 1st day of last menstral cycle. The strictest definition would be13 wks, 2 days 8 hrs. By tradition 13 completed weeks will suffice.
13 1/3 weeks. Is the 1st trimester, so 13 wks and like 3 days starts the 2nd trimester.