Lucky you...Here in Oregon it's been so wet all spring I've only started putting my garden in this past week, and I'm still not done.

Here's how I do my Bsprouts for example:
The variety of Bsprouts I grow take 120 days to begin to mature, and I need about a month to get the full harvest from my plants and they taste better when they have been exposed to some light frost. So I count backward from our typical first frost (Oct 31 in my area) 120 days. (I will need to cover them with row covers towards the end of harvest if we have a chance of a freeze). So I figure I need to have my transplants ready to go in the garden around July 1 (120 days from July 1 to Oct 31). I know that It takes about a week for seeds to germinate and it takes me about 4 weeks from germination to grow a seedling to the size I want to put out into the garden, and I need a week to harden the plants off, so I need to start germinating seeds 6 weeks prior to set out date of July 1, so that would mean I will be starting germinating seeds for Bsprouts this weekend.
I cover my transplants with row covers when I put them out for two reasons. First to shade them from our hot sun. Second to protect them from all the pests that are prevalent in mid summer.
You can adjust the start date by how long your selected variety takes to mature, and you can do succession planting to have your broccoli and cauliflower mature over a longer period of time. I will start 6 cauliflower seedlings all at the same time, but transplant them into the garden one a week over a 6 week span to stagger the harvest. This way I have one head a week for 6 weeks. I plant broccoli in batches of 6 plants a week (I like broccoli more than cauliflower) because one head of broccoli is not enough for a meal in our household whereas one head of cauliflower is.
I have never had any luck with direct seeding cole crops for some reason. But I have been told that if you are going to direct seed in summer, you need to sow the seed at twice the normal depth in order to get the seeds down to the level of moisture in the soil.

For your area you will want to set out cauliflower about July 25th. Broccoli can be set out then and some a little later to stagger the harvest. This means setting out plants at that time. Plants are better than direct seeding as flea beetles will eat the tiny seedlings out in the garden.

I start seeds outdoors on a picnic bench for these fall crops in plastic pots. I take them under roof if a bad storm is coming. Otherwise they stay out there until transplanting. In that hot weather the plants do need special care in watering...both in the pots and after setting out.

Finding the right planting date can be hard! By networking with local organic gardeners, I discovered that mine is Father's Day (around June 20). Started indoors then and set out in late July, the brassica seedlings benefit from summer's last blast of heat, which they love. I use nylon tulle row covers held aloft with hoops to keep the grasshoppers off of them. Broccoli and cousins are ready to start harvesting in mid to late Sept continuing for weeks. Good luck!

You could also look for Elliot Coleman's Four Season Harvest book. It has good charts - very much like NCDirtDigger posted, with some tips about how things grow slightly more slowly in the fall (the sun is lower, so you get less sun).

I got the book last year, and it worked out pretty well for my first time. It's tricky to figure out how to plant stuff in my small garden when the summer veggies are at their peak, though.

We made a cold frame just for lettuces with a sliding glass door I found for the glass - really insulated. The problem was, the only good place for it is also where the snow plow dumps driveway snow. Before the first big snow, (December), my hubby got worried the 8+ feet of snow in the pile by spring would break the glass, so we harvested everything. We cut it a couple of inches above the soil level. The snow must have been a good insulator, because most of it came back. We're still cutting from what I planted in the fall - pretty nifty!

Wow - I didn't expect so many responses so quick. Thank you all.

Okay...so my last frost date is October 15. My romanesco broccoli says 80 days to produce, which would be about July 26th. It also says seedlings emerge in 10 to 21 days. So if I want my seedlings to be 3 weeks old when I set them out, I need to sow them about 6 weeks before. So if I'm setting them out about July 26, I need to sow them June 13? Is that right?

I would say to start the seeds about June 25th.....In really warm weather broccoli can come up in as little as 3 days.

Romanesco is very very slow, takes a ton of room and is somewhat finicky. Hedge your bets by including a sure thing in your lineup, like belstar broccoli or a fast cabbage. Good luck!

I usually have two main problems when planting fall crops. One of them is insects. By the time you plant the fall vegetables, all of the insects have been out and they can destroy small plants quickly. Last year I had lots of grasshoppers. They ate most of my brassica seedlings. Also, you will probably need to spray Bt regularly or the caterpillars will slowly eat your plants.

The next problem I have with fall vegetables is watering. It can be hard to keep enough moisture in the soil to allow the plants to grow well. I've seen my fall brassicas just sit there a few times in the fall when they had enough water to live, but not enough to grow well.

It should only take 4 days for the seedlings to emerge when temperatures are 70F inside. Most of mine take 4 days; a few are up in 3 days and I've had some come up in 2 days inside.

Thanks again for all the answers and comments! I really appreciate it. I plan on both regular broccoli and cauliflower, as well as the romanesco. The romanesco is a lark...it's just so cool looking I couldn't resist when I found the seeds, but it was too late then to start them for spring. I have a shady yard with only a TINY sunny spot for the garden, about 7 1/2 x 3 1/2, but I successfully grew two broccoli and two cauliflower across in a row one spring. My biggest problem actually will be finding a space for the seedlings...It will still be full in July. I can't believe I didn't think of this before! Right now I have pole beans at one end, then a basil, rosemary and oregano. Then two bush tomatoes. Then onions, parsley and carrots, then spinach, then lettuce. Not really square foot gardening, but close. Maybe I could put them in planters? Darn...all this planning and I'm now seeing the problem of planting space for the first time.

I have had the same problem and I finally figured out that I need to start way earlier than I ever thought--so the advice above is good.
I had always tried to do Brussels sprouts and similar things from available transplants, but the garden centers here get in the fall veggie transplants way too late, IMO, or at least for my planting conditions. I think the young plants must need some of the warmer weather and more intense sun to get mature enough before the cool weather. So this year I am going to start my own from seeds in mid-summer.

Same for some of the cool-weather greens--with the exception of collards, which grow like weeds, I need to start some of the other types earlier--when you're still thinking, man, it's hot!-- to get good growth going and then they tend to "hold" pretty well in the cooler weather and even partial shade and you can harvest them here and there. But if I wait until September, they aren't coming on strong until early spring! Perhaps if I had everything in full-blast sun from dawn to dusk that would not be a problem, but probably in several of my veggie areas the light angle and intensity starts to fall, so again, they miss out during that key early growth period.