You are going to need to post a pic...seeing the tepals,anthers etc are important to make a correct ID...Jeanne

There's a couple that come to mind with flowers that size, a very early spring bloom season and leaves that fit (somewhat) your description. It is possibly a white flowering form of Clematis montana, the anemone clematis or alternatively, one of the x cartmanii hybrids, like 'Early Sensation' or 'Avalanche'. If the latter, it won't be hardy for you in zone 6 :-((

You can Google images of montanas to see if it matches them or I've linked to a photo of 'Early Sensation' for comparison. ES and other x cartmanii's have very distinct foliage - quite unlike most other clems - so look closely to see if it matches.

Here is a link that might be useful: {{gwi:583324}}

Nurseryman's Exchange grows cartmanii hybrids for mass market outlets as potted gift plants.

Avalanche was the first one that came to my mind.