Students always get excited when a holiday rolls around, especially if it falls on a day they're in school. Although the holidays do make teaching a tad more difficult (especially if a party is involved), they do lend to discussions about family, friendship, heroism, and love.
I typically spend the days leading up to a holiday reading picture books to my students. This is a good way to teach various reading strategies and skills with some literature that your students are excited about.
Below is a list of books, targeted at various grade levels, that would be appropriate to read for Valentine's Day. You can choose one to read every day, check out a bunch from the library and let students choose to read them throughout the week, have a parent volunteer read one to the class, or even have students read to younger grade levels in the school.
If you'd like to read a short summary of a specific book, just hover over the bookshelf and it'll take you Amazon. :)