This date is not precise, but can't be more than two days off.  It seems to be nine days after the arrival at Canaples—but the count of days includes a period of "about a week" and hence is not exact.  Luynes later says [Chapter XV] that this day is seven weeks before April 30, which would be March 12, but again the statement is rather vague.  And as argued elsewhere, the date should be two or three days before the full moon on March 18.