If I'm right, "When you play..." triggers only once (when you've met all costs and have played the card), while "Each time you play..." can feasibly trigger more than once (each and every time you meet the costs and play one of the appropriate cards).
So, if you were to word your card as "When you play Gladden Fields, you may play a character to etc.", you'd only have the opportunity to use Gladden Fields' text once during the whole game. However, if your were to word it as "Each time you play a character to Gladden Fields, you may etc.", you could use it more than once, if you'd the characters available to play.
Also, do you intend any player to be able to use this site's text, or only the player who played it?