Happy Valentine's Day! Some tips for punctuation and capitalization of this loving holiday:
1. In a greeting (such as a card), cap all three words: Happy Valentine's Day.
2. Show possession to the singular proper noun Valentine--the day belongs to him: Valentine's Day.
3. Cap Day in the holiday because it is part of the holiday's name (as opposed to day in Christmas day in which Christmas is the name of the holiday, not day): Valentine's Day.
4. You can just call it Valentine when appropriate, but remember that Valentine is a singular proper noun, so in other contexts, do not plural it before showing possession:
a. I'm making Valentine's cupcakes.
b. I got a Valentine card.
5. If you are calling cards Valentines, keep the following in mind:
a. Still cap it--any time a proper noun element is used, it retains its capitalization: I got a Valentine this morning. (In this case, it is sometimes called a proper adjective--an adjective that is a proper noun in its non-descriptive states.)
b. Just plural it with an s (not an apostrophe s): I got some Valentines at school today.
Most of all, enjoy your Valentine's Day! :)