Athosanswered grow question 6 years ago It all comes down to what is day 1. For some people it is when the seed break soil, for others when they open the first set of true leaves (me included) and there are some others that count day 1 since the very beggining of the germinating process. Pick whichever you like.
I think most people use one of the first two options.
Also keep in mind that week 1 is day 1 to 7, week 2 is day 8 to 14, week 3 is day 15 to 21, and so on and so forth. So day 13 would be only week 2.