Make sure to point out the Valentine decorations as they start being hung in stores and on homes. Then buy or borrow some simple Valentine's Day books for toddlers, like "Where Is Baby's Valentine?" by Karen Katz, or "Elmo's Valentine" by Stephanie St. Pierre. Point out the hearts in each book, as well as that hearts are used to symbolize love.
While teaching anyone under the age of 5 to cut out a heart is usually a frustrating and fruitless endeavor, there are still lots of heart activities that toddlers will enjoy. Start by going on a shape walk, where your toddler searches the house or grocery store for hearts. You can keep track by making tally marks in a notebook, or help your toddler to count on her fingers. Encourage your child to make a collage of hearts (precut by you) with construction paper and a glue stick. Help your child draw a heart (guide his hand) and then let him try to color it in. As you draw together, be sure to state what you are doing ("Now we're making the curved part, now we're reaching the point").
Have your child sort shapes by deciding which ones are hearts and which ones are not hearts. You can cut out shapes and then use a poster board with a line drawn down the middle to sort into two sections: "Hearts" and "Not Hearts." (You can have them glue the shapes down or just move them to the right areas.)
Encourage your child to make Valentine's Day cards for friends and family members. Discuss what shapes to use. Help him draw or glue hearts onto cards.