Befriend an Animal
In order to receive any special acknowledgement of Valentine's Day in the game, you will have to befriend an animal of the opposite gender. Befriend the animal by speaking to her every day and presenting her with gifts she likes. Befriend her as soon as possible so that you are on good enough terms to exchange Valentines for the holiday. It is possible to befriend an animal enough to receive a present within a couple of weeks before the holiday if you know the "send a Valentine" cheat.
Send a Valentine
In order to receive the limited Valentine's Day chocolate heart candy box on Valentine's Day, you will have to plan ahead. Beginning on February 7th, real time, you will have one week in which to send out a Valentine to an animal in your village. In the letter, write, "Will you be my Valentine?" Send the Valentine no later than February 13th. The animal to whom you send the Valentine must be the opposite gender of your player character. Choose the animal with whom you think your character has the best relationship.
Brewster's Greeting
Another event that takes place on Valentine's Day that will only occur if you know what to do is a unique greeting from the non-player-character Brewster. Head to The Roost, the cafe in the lower level of the museum in town, on Valentine's Day. Purchase coffee and Brewster the barista bird will say "...Happy Valentine's Day" in lieu of his typical greeting.
Change the Date
If you are unable to play on Valentine's Day, you don't have to wait a year for the event to appear again. Speed through time or rewind time in order to trigger the Valentine's Day celebrations in your game by adjusting the internal clock on your Nintendo Wii system. Before you select the game, select "Wii Settings" and "Calendar." Change the date to February 14th and select "Confirm." Load the game and the game will act as if it is Valentine's Day. Don't forget to change your calendar back if you want to keep game play accurate.