Section 8 exact date to issue

Hi, i literally cant find the right answer to this crucial question. Most websites say you can issue a section 8 if tenant hasnt paid rent for 8 weeks. Does it have to be full 8 weeks or 2 months because i read somewhere else that just 1 day after second rent is due already counts as 2 months arrears. Our tentant used to pay monthly on the 14th of every month. Last payment we recieved was 14th of December.
So when can i issue a section 8? Feb 14th or March 14th?
Thank you

Hi Tony, I recommend looking at the grounds, here