When/how to start using a greenhouse...?

I am in zone 5 and have a boatload of vegetable seedlings and not enough windows with good sunlight in the house. so I purchased one of those plastic greenhouse kits (just a shelf covered with clear vinyl, basically) and I'm wondering when it's safe to set some plants in it? I can sure bring them in at night if I need to. it's been bouncing back and forth between highs in the 30's and the 50's during the day for the last couple weeks, and it's been a low in the mid 20s at night, usually 30-something. thanks!!