Greeting card companies don't invent the holidays and much as they exploit them. For instance, Saint Valentine's Day has been around since the 5th century - since Valentine was an actual person. However, the greeting card companies took the feast day - when the tradition was already established to send love letters on that day- and made it into what it is today. You can see some early examples and read more about it. Here is the oldest known Valentine from the 15th century.

As for Santa Claus, Snopes has it already as false.