We know from the first paragraph of Chapter XXV that the attack was in March.  Also, according to Isotta, the attack "took place on the night of Monday" [Chapter XXIX].  (In Chapter XXX Isotta says that the attack was on a Tuesday; I'm taking her earlier statement as the more accurate—memory fades with time.)  Finally, Vendramin finished sounding the canals "within a fortnight of having set his hand to the task" and hired his bullies the same day.

So the attack was either March 20 or March 27, with the former more likely.  To establish the date positively, we note that ten days following the attack Vendramin says that Easter is "a month hence".  Such a statement makes no sense on April 6, ten days before Easter, but is reasonable on March 30 when still in the preceding calendar month.