You did not say what sort ,the small flowered Stelata type are earlier than the large flowered ones.I am in the West of England and rule of thumb,March for the small a month later for the large.The farther north you are the later that will be

Wouldn't expect to see flowers on our Stelata until mid April...

Nore would i if i lived where you do, I have toured all your alpine nurseries from top to bottom in April.

Almost sounds painful Bjs :-)