From Nookipedia, the Animal Crossing wiki
Valentine's Day in the Animal Crossing series is not as significant as the other holidays, especially because there are no furniture collections for it. There are some activities the player can participate during the holiday.
In the original game, the player receives letters from various villagers. The more the villager likes the player, the better the gift will be.
A week or two before Valentine's Day, villagers begin to hint that they are going to send chocolates to a special someone. Pete, the mailman, also advises everyone to clear their mailboxes.
On Valentine's Day, the player will receive letters from all of the animals in the town of the opposite gender. Each of the three male or female personality types can write two different kinds of letters. Thus, each gender can receive up to twelve different Valentines from their villagers. Each villager will attach a gift to their letter. What the player receives in the mail depends on their relationship with that villager. Shirts are given for fair to poor relationships and furniture is given for good relationships.
In Animal Crossing: City Folk, the player can send a letter to a villager of the opposite gender saying "Will you be my Valentine?" a week before Valentine's Day. On the holiday, they will send the player a Chocolate Heart. If not, the player receives a box of chocolates from the villager they are most friendly with. In addition to this, if the player goes to The Roost, Brewster will serve them hot chocolate and wish him or her a happy Valentine's Day.