Thay are called florist hydrangeas, a term that suggests where they once came from. And yes, they were for sale around Mother's Day.

Florist hydrangea plant labels usually do not indicate much, like the name of the variety, winter hardiness, estimated height/width, etc. But what is great..... yours appear to be hardy in your zone. Many florist hydrangeas begin to have trouble with winter hardiness below USDA Zone 7. As a result they may not bloom at all or bloom poorly in Spring. All growth above the ground dries out and the shrub returns back from the base or crown. I am glad to hear yours is doing so well!

Yay, I guess I lucked out. I'll try looking them up on the net now that I have something to go by. Thanks for all your help.