Transplanting seedlings versus planting larger plants (timing/dates)

I have been using a gardening calendar website that helps you know when to plant things based upon your location. http://www.gardenate.com/

It tells you for example with Broccoli (in my zone 7), when to:
-(March) Plant undercover in seed trays & transplant in 4-6 weeks
-(April) Plant out seedlings
-(May) Plant direct in garden where they are to grow

Now, help me figure this out. For April, does that mean plant out the itty bitty seedlings you have just grown indoors, but in May, if you are just getting started, plant the bigger plants you bought from the garden store? What is the difference between April & May's instructions?

I'm in the MidAtlantic region
http://www.gardenate.com/plant/Broccoli

It's April, I'm 1 week away from my last frost date, and have nothing in the ground yet, & no seeds started. So for the ones I know I needed to have seeds started already (and I'm late to the game)--I'm just going to buy the plants this year.

So for example in the case of Broccoli, do I buy plants now at my garden store and put them in my 4x4 bed? Or do I wait until May? There are other crops I need to do this with (peppers, tomatoes for ex.), but this will give me a good frame of reference for what I need to be doing.

Thanks!

The way I read the chart for Broccoli, for the Mid-Atlantic Region

Start seeds indoors (about 10 weeks before your last frost), then 4 - 6 weeks later (about 4 - 6 weeks before last frost), transplant your seedlings into the garden. These seedlings should be big enough. Harvest about two months after transplanting.

If you were unable to start your own seeds indoors, purchase transplants now and put them out soon (now) so they will mature before it gets too hot. (about 60-70 days from transplants for most varieties)

They are suggesting that you plant more seeds directly in the garden in May so you can harvest in August. That would have your broccoli maturing during the real heat of the summer, and broccoli is a cool weather plant I am not sure I agree with that at all.

If you are looking for a fall harvest, you could plant seeds about 3 1/2-4 months before your First Fall Frost. The seeds would germinate and grow during your warmer months, but not mature until it starts to cool down.

Hope this wasn't too confusing. I don't always explain things well.

In general, broccoli can be started indoors from seeds about 12 weeks before your last frost date.
Then plant these same seedlings in the ground outdoors, about 5 weeks before your last frost date.
Transplant when they are 4-6" tall and have 4-6 true leaves and harden them off about a week before planting outdoors.
Broccoli is a cool-weather crop and can take some cold with a little protection, like covering with an old towel, sheet or heavy row cover when there's a freezing nite expected.

In my blog I post how to grow different crops and in a pinch, if I want to renew my brain with info, I re-read the article. I also have tips, like planting them a half inch deeper than they were in their seedling pot or cellpack.

For a fall crop, start seed in the garden in late spring. They can take some heat when they are germinating and actually seeds prefer being warm.

Again, if its too warm outdoors, seed can always be started indoors for fall crops if you prefer.

Good luck with your garden.