It must, therefore, be written "Seven [and not six] days shall you eat matzoth." (Ibid. 15) "Only on the first day": This makes (the eating of matzoth on) the first day mandatory and the others optional. You say this, but perhaps (the intent is) to make the first optional and the others mandator? It is, therefore, written (18) "On the first day, on the fourteenth day of the month, in the evening shall you eat matzoth," whereby Scripture fixes the first day as mandatory. It is not the second assumption, then, which is to be accepted, but the first.