I say go for it............for every little holiday.....
My two don't have a specific outfit for St. Patrick's Day, but they will wear their green hair bows for sure....and possibly an outfit that has some green on it. I wish we did more for St. Patrick's Day...but we normally don't.
As for Easter, I am just using Spring colors for their outfits....BUT that doesn't mean I wouldn't buy something if I thought it was just too adorable to pass up...I too like lambs...or pastel flowers/grass...etc.
AND...I of course, would want something for the boys too...that way we can match.