I grow several cultivars of Cerastium, including ‘Olympia,’ ‘Yo Yo,’ and ‘Silver Carpet,’ as well as the species, Cerastium tomentosum. ‘Olympia’ has been performing the best in my garden, holding up better in the summer, whereas the others literally melt in the heat and humidity, leaving a mat of fried foliage. I wonder if the fact that my ‘Olympia’ gets an additional hour of afternoon shade might be the reason for its better performance? In any case, I’d recommend ‘Olympia’ over the others for its larger and more abundant flowers.
Last year, ‘Olympia’ bloomed from May 20 to June 11. It looks ratty in the summer, but giving some protection from afternoon sunlight might help. Also, don’t give any supplemental water as it thrives in dry conditions.