Lazzaro and Paolo were married on Christmas Eve. Working backward through the events of that momentous week, we find that Paolo was poisoned at supper December 20, which in 1502 was a Tuesday. But the text states that this supper took place on a Sunday, and that the day before that supper, when Paola repulses Ramiro's unwelcome advances, was in "early December" [Chapter XIII].
The simplest explanation is that Lazzaro's memory was slightly off—possibly unhinged by his grief over Paola's "death"—and that Ramiro actually hit on Paola on a Saturday evening earlier in the month. Lazzaro then conflates the next day's supper with that of the poisoning.