I found this online and it looks like a great tool in knowing nectar sources in your area for specific months. The significant sources of nectar are bolded in orange.
Click on the map picture on the main page, then click on your state, and there's your list, beginning with the earliest bloom month and ending with the latest bloom month. Right now for my region, New Hampshire, it shows significant sources are red clover, locust, sumac, basswood, in July will start goldenrod, and in August will start aster. It doesn't show dandelion (or maple) as a significant source in April, which surprises me, I figured dandelion would be a significant source. (It's a source, just not bolded in significant.)
I didn't realize NASA was interested in honeybees.