This is a tough type of garden to establish for a couple of reasons, and I don't want to be overly discouraging, but I do want you to do a reality check. I do think that in your area it may be late to start from seed for this year. But you can do research and find out what is available for your area and do the needed soil prep if you decide that this is the way to go.

IME unless you are careful to get actual local wildflowers you often get self-seeders that may be wildflowers somewhere in the world but aren't native where you are and so can be problems, either through spreading everywhere into your yard via seeds or dying since the conditions aren't right. If you decide to go this route, find a reputable vendor or producer who lists the actual flower species and a mix that is made up of flowers that are native to your area or at least free of plants that can be problems and is suited for the site you have as far as soil type and moisture levels. A couple of times earlier in my gardening life I used "wildflower" seed mixes that gave me aggressively self-seeding weeds that I am still trying to remove permanently from my yard.

You mention a hill, and seeds tend to flow downhill with water, so you may end up with many more plants at the bottom than at the top.

Another issue is that you need to do quite a bit of soil prep for these to be successful and not look like an unmown weed-patch. You will want to remove the plants and seeds already there via solar sterilization of the soil and/or a short-term herbicide. Currently I assume something is growing there, and there is a huge bank of seeds already in the soil that may include some desirable plants, but may also include undesirables. So whatever you seed will be competing with other plants (even if you sterilize the area, a few will survive IME), and you will need to know what the seedlings look like of both the weeds already in the soil and the wildflowers you will be sowing so you can remove the ones you don't want. This type of bed involves a lot of work the first couple of years until the wildflowers have filled in and spot-weeding afterwards. It will also probably need mowing at least once a year, after the wildflowers have gone to seed, either with a string trimmer or old-fashioned hedge shears. The east coast doesn't have native fields - they have to be created by removing woody plants, so if not mowed, the seeds of blackberries, white pine, wild cherry, and maples will sprout and grow into bushes and trees. In the midwest, prairies are managed by controlled burns, but I can't imagine that would go over well on LI. ;>) Edited to add that it was kindly pointed out to me off list that Long Island is one of the few areas on the east coast (along with other glacially deposited islands and Cape Cod) that has native prairie . . . so you may not have the issues with woody plants that I mentioned above.

IME, the easiest way to create a meadow-type area is to create a meadow garden of low-care, flowering perennials and grasses. You can start plants in pots from seed (check out winter sowing) or buy small starts, and do bed preparation so that there will be less competition there. Cut everything down once a year in the winter. Decide if you want spreading plants that will duke it out amongst themselves to fill the space (examples include Joe Pye weed and bee balm/Mondarda or a bunch of well-behaved plants that will pretty much stay in place or perhaps self-seed a small amount (examples include Arkansas bluestar/Amsonia hubrichtii and Baptisia).

Susan , I think nhbabs makes some great points . However I think you could sow seeds till the end of may just fine ! Last year I did what you described. I had a front area that was sloped , that was dug up by the utility company . I spread wildflower mix and zinnia seeds . It rained really heavy and I did lose a lot of seeds , but still had a great display !! Oddly my zinnias were the only thing that bloomed . I'm sure the wild flowers would have bloomed this year but I decided to have the area tilled again. This year I did zinnias and sunflower seeds . I'm so excited for them!! My advise is give it a try !! My seeds came from grow organic .com the zinnias attracted soooo many butterflies it was amazing .

I have a small slope garden and it's a bit of a challenge, but who doesn't like a garden challenge? I say go for it and see what happens.