Valentine's Day is a holiday that is celebrated in Bikini Bottom.
For the citizens, it appears that it depends on who the citizen is.
SpongeBob gives his most of his friends generic hearts and other gifts. With his best friends, he gives them special things like a chocolate heart for Sandy and a giant chocolate heart balloon for Patrick.
Patrick once was planning to give SpongeBob a rock that was in the shape of a heart. However, he thought SpongeBob was trapped inside, so he destroyed it.
Sandy once gave SpongeBob an acorn that was in the shape of a heart with an arrow in it.
Other citizens spend there day with their friends and loved ones at the Carnival.