Red Letter Day was a virtual representation of one of Ezio Auditore da Firenze's genetic memories, relived by Desmond Miles in 2012 through the Animus.
Ezio Auditore received a contract from Niccolò Machiavelli to assassinate two of Cesare's new couriers.
Ezio assassinated the couriers and thereby prevented them from delivering their messages.
Ezio assassinating the first courier
Ezio assassinating the second courier