Valentine's Day in Animal Crossing is not as significant as the other holidays, especially because there is not a furniture collection for it. However, there are a couple things the player can do to celebrate the holiday:
In the original game, the player receives letters from various villagers. The more the villager likes the player, the better their 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 (which is a smart idea).
On Valentine's Day, you will receive letters from all of the animals in your 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 you receive in the mail depends on your relationship with that villager. It pays to be nice and run lots of errands in the early part of February; shirts are given for so-so to poor relationships, but furniture is given if you and the animal are good friends.
In 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 you 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 you hot chocolate instead of coffee and wish you a happy Valentine's Day.