Valentine Event
The Valentine's Event is an event introduced by Riako on 12th February 2014. In this event you are given the option of sending special cards to other users with messages.
At the beginning you were only able to send and receive cards via Ducklett Delivery but over time it got more advanced.
Back in 2014 if you managed to receive at least one card before the event ends, then you had the chance of receiving a special event Pokemon.
Over the years it became more of a chellange to send out cards especially if you would like to try to get the newest event in its shiny form!
Valentine Cards
They cost 50 
Sweet Hearts each and can be sent either anonymously or not. In addition to that, messages may be sent to your closest friends or complete strangers.
Sweet Hearts
These are obtained through various activities in PokeHeroes, namely Rumbling, Interacting and sending a Dream World plushie. (Note the minimum is 10 Dream Points).