It was twelve days after Don Pedro's arrival that Margaret summoned Gervase to send the ransom note, which he did that very day [Chapter VIII].  The reply came eighteen days later, and the abduction occurred the following day.  This reckoning places the abduction on October 27.  However, we know that the day after the abduction, when Gervase set sail with Oliver Tressilian, was Sunday [Chapter XII].  So a day is overcounted here somewhere.