Carnevale
The Carnevale is an annual event held in Venice, Italy. It is essentially a carnival unique to Venice where people wear masks.
Ezio obtains a mask and has his Plain Cape embroidered during this event to remain anonymous. It is during this event that Ezio retrieves the Golden Mask to gain entrance to Marco Barbarigo's personal party. It is also in this party that Marco is assassinated.
Despite being featured in the trailer, Carnevale itself has no major role beyond the assassination of Marco Barbarigo.
Attire
During the Carnevale, Ezio wears a Carnevale Mask, given to him by Leonardo da Vinci. The mask makes him permanently anonymous and prevents any increase in Notoriety. Ezio also dons a Plain Cape that is embroidered, but does not affect gameplay at all.