Answer: Most perennial seedlings will tolerate cool weather if they are sufficiently hardened off. This means you need to gradually acclimate them to life outside, over the period of a week or two -- this is true even when the weather is already mild outside. Set them in a spot that is out of the wind and in the shade. Gradually move them so that they are in the morning sunlight a little longer each day until eventually they are in the sun at least half the day. This allows them to get used to real sunshine, a little wind, and so on and decreases the chance of shocking them when they are finally planted in the garden. If you have a coldframe, this would be the time to use it since you can cover them easily on frosty cold nights or shelter them from driving rain or hail during the process yet leave them open during nice weather.
Most winter hardy perennial seedlings can go outside in the garden around the last frost. This year, the weather is a bit unsettling and it is no better to expose them suddenly to extreme heat than it is to extreme cold. Too, the frost date is an average and some years run early and some run late, so you have to use your best judgement and hope for the best.
Many annual seeds can be sown within a week or so either side of the frost date, and others can be sown quite early. The seed packet should give you some guidance. Usually they germinate in response to both moisture and soil temperature and do it when things are "right". Since things seem to be running a bit early and warm this year, you could probably try some and see what happens. If there is still another sharp frost or hard freeze there will still be time to replant.
Have fun with your seedlings!
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