If you are growing tomatoes, peppers, or other heat loving annuals, they will suffer going outside now. They have to wait until at least mid May for our region. Cool weather annuals will be fine going in now.

Back in early march the wife and I bought a portable greenhouse from the depot, and I have had my seedlings outside pretty much since. I started most of my seeds in the middle of February(needed to get my mind off of the 70 plus inches of snow this winter), and a few weeks ago I planted several of my seedlings (Lavender Munstead), and while they are growing kinda slow, they are doing well!

I'm in SE PA, and I put some of my tomato plants out in early April - first week I believe. They were already hardened-off with decent size root systems since I started seed in February. I planted them deep, bottom 6" of the stem was buried and now they are 2' tall some with flowers.

I woudln't always recommend putting tomatoes in the ground as early as I did, but I do advise getting them in as early as the weather will allow. I grow some later maturing varieties (90 days+) and I want to ensure a good harvest. Sure, I had to protect them from a few cold spells we had and two frosts, but to get tomatoes by early July that's the price I pay (My first ripe tomato was July 7th last year, hoping for tomatoes by the 4th this year).

In general though, I never plant hardened tomato plants any later than mid April, mid May is playing it a little too safe in my opinion - since later maturing tomatoes can use any head start you can give. If you don't want to gamble a little, then put out mid May, but tomato plants can take the occasional nighttime lows down to the low 40s (and even sometimes high 30s) without too much problem - provided they are healthy and properly hardened and the soil temps are in the 50s.

It's still a little too early for peppers now, but due to our recent heat wave, if you had them planted in containers (like I did), they should be taking off right about now. I just put them outside during the day when warm and take them into the garage at night, until nighttime temperatures become more favorable. Peppers and Eggplants can't take the cold like Tomatoes and Cucumbers can... Just my 2 cents.

AS far as the comment about hardening off the seedlings. You should generally start that one to two weeks before you want to plant. If you were growing them on a window sill, one to two weeks before you want to plant, you should leave them outside in a shaded area with very little direct light and wind on days that are warmer and take them inside at night - do that, gradually increasing the sun and wind exposure until you plant them. They should be outside fully exposed for a couple nights by the time they are ready for the garden.

Sometimes you can get away with less hardening time like you mentioned, but that's another gamble you're taking. I think we're supposed to get high winds this weekend, at least in my area, so if they're not used to getting whipped around by wind, they might just snap.