Carnival Month is also known as Masquerade Days! This happens throughout the month of February and lasts the whole month. During this month you can go to the NPC Stan in Venore and purchase costume bags! You can also purchase party hats and party trumpets from him but this is not where the money is at because these are easily obtainable during other events.
(He's located in Venore in Hugo's shop just south-east from the main depot)
Common Costume - 500 gp
Uncommon Costume- 1000 gp
Deluxe Costume-1500 gp
These bags will last you 5 hours and can turn you into a creature depending on the bag you purchase. A lot of players will purchase these for months on end. The reason why they purchase them is to run across Thais as a Banshee for example so it's funny to see!
The way I profit off of this event is purchase as much bags as I can to sell for months later after everyone already uses their bags. This is how you can profit by simply placing them for sale on Tibia's market to make some coin.