"Seven days (shall you eat matzoth"): including the first day of the festival. You say this, but perhaps (the meaning is) excluding the first day? It is, therefore, written (Ibid. 18) "until the twenty-first day" (i.e., since the seven days end on the twenty-first day of Nissan, the first day [i.e., the fourteenth of Nissan] must be included in the count). And if (it were written only) "until the twenty-first day" (and it were not written "Seven days"), I would exclude (the twenty-first day, which is) close to the twentieth (and I would read it as "until [but not including] the twenty-first day.")