Buying dill seed is something you'll probably only have to do once, since it's a very efficient self-sower and you can save seeds from the seed heads, too. I grow dill in a band one foot wide, 25 feet long, as a fence on one side of my plot. If I just let it self sow I end up doing some transplanting, which isn't ideal because of the tap root that the dill sends down. So with last year's saved seed I've planted 2 parallel rows about 8 inches apart (they would actually like more space than this) and I'll eventually thin the plants in the row to about 6-8 inches. Before then I'll have lots of thinnings do use as dill weed.
As is true with any plant that reliably self-sows, it's very resistant to frost in the spring, so you can plant it now without much concern (there are already planted seeds in the soil after all, dropped from last year's seed heads). If your interest is primarily the dill weed instead of the seeds, plan on sowing it at least a couple of times during the summer so you always have a fresh supply. After the plant flowers the fronds deteriorate or even dry up. There are dwarf varieties and varieties best suited to the greens, but you'll probably have to plan ahead and find them in catalogs. Look in Johnny's Seeds.
Dill goes well with salmon of course, but my favorite summer use for the green dill is in a dish of rice, summer squash or zucchini, onions, black pepper, and dill. Topped with sour cream. *sigh* Will post something like a recipe when the zucchini comes in.
I also use the seed heads in half sour fermented pickles (recipe will follow soon). The picture above shows the interior of a gallon jar with garlic, a little pickling spice, and a dill seed head, before I've added the cukes and brine. The great thing about growing your own is that you can pick the seeds at just the right point for pickling, which in my view is while the seeds are mature but still green. The picture below shows dew on a patch of dill. Morning in the garden doesn't get much lovelier than that.