The main thing is to avoid exposing the seedlings to temps below 45 degrees. They will love going out on sunny days as long as they are protected from wind.

I also need my onions out of the way before I start tomatoes and peppers in mid March. There always comes some crazy cold in April when they all have to come inside, and the bathtubs are full of seedling flats.

In the Denver area most nights are still going to be in the teens or twenties and certainly we haven't seen the end of even colder nights. The little greenhouse isn't going to retain enough heat most nights for onions until later in the season. I'm not going to move my onions out into a cold frame until the nights are more consistently in the 30s. Not that there's anything consistent with Colorado weather.

thanks guys for the advice! Think I'll move the onions outside when the days are warm and sunny and then bring the babies inside at night.