Search your average FROST FREE DATE. That's when most people start outdoors. But if you only have a few plants I would wait until couple of weeks after that date, (if you can). If you have lots of plants (or make clones) you can start earlier by planting one each day starting 2-3 weeks before that date; sure some will get hit with night-time frost and die, but you will have the earliest planting and most will make it. Look into extending your growing season by putting up small plastic hot-houses over your plants, (the trick to this is not to cook them, the plants have to be close because you will have to put the hot-house on before night, but you might have to take it off durring the middle of the day too then back on again before night).
The last day to transplant depends on the kind you are growing, you need to have enough time for it to finish before the FIRST FROST DATE. Find out when yours is, and count back however many weeks your bud needs. Hothouses help in the Fall too!
Also mulching with large rocks or putting out black containers of water. Both of these will collect the sun's heat and give it back to the plants near it.
Don't forget to "harden them off"!! Which means you have to gradually get your plants used to the outside or they might freak out a little. Just put them outside in the SHADE (never in direct sunlight) durring the daytime and inside at night for a day or two.