You want to start about a week before you plan to plant out.

As to when you would plant out, a week to 10 days after your last frost date and after you check the long range weather forecast carefully. In my area that is April 23 but I don't know what yours may be. That info is available on the web.

Dave

I try to harden off as much as I can throughout the spring. The earlier I can do it the better. If I can get my seedlings in a little direct sun on a warm day, I do that for a short period. If I have several warm days in a row, I incrementally increase the sun duration. I also leave them out in the afternoon as it cools down. The thing you have to watch is cold temps (below 40 makes me nervous), and sun scald. I worry about sun scald more than cold temps. Sun scald is a problem when you have the plants under indoor lights. So if you sprouted under indoor lights, you have to take it slow. If you are able to get your little seedlings full sun when they first come out, they seem to be more resistant to sun scald.

I'll send my seedlings outside any time the weather allows for it, even if it's much earlier than the planting-out date. Real sunlight is going to be better than my grow lights any day, and outdoor breezes will keep them sturdy.

If outdoor days are followed by too many indoor days, I act like I'm starting over the next time they go outside.

I get mine out as much as I can during the day. Seems like real sun and a little fresh air makes them stronger and healthier.
Just be sure to start slowly and gently. No direct sun and use a wind break first of all.
I bring them in at night unless temps are above 45 degrees.
I'm lucky enough to have two seedling racks with three shelves leftover from my medical clinic, used for my supply closet. I use one indoors with lights and outdoors which I can roll under the porch r out into direct sun.

As most posters above say. A simple guideline is to start hardening them off on the last frost date, then plant them 1-2 weeks later depending on the 14 day forecast. Start in shade. You can start sooner if you wish as other posters have mentioned, but the benefit is minimal over the long term, and you may have to start from scratch if you get a daytime cold snap. A fan on the seedlings while still indoors helps strengthen them up for transplant though.