I buy my annuals before Memorial Day weekend just to avoid the crazy crowd rush. But I usually wait until then to plant them - and most go in planters and pots by the front of house so are more protected from cold. I just don't trust the weather here before then! The latest killing frost I remember here was a June many years ago - I remember it clearly because I was driving to friends' wedding and all the corn and soybean plants that had come up were frozen in the fields. Sad for the farmers especially.
I have had springs where daffodil buds froze on the plants. It was very sad....
Ginni