If you mean tender geraniums (i.e. Pelargoniums), then when all danger of frost has passed in your area. When is that? Who knows! Try with just a few when you think it's getting warm enough (so you don't lose the whole lot one night) but keep an eye on the weather forecast.
You can also move them outside during the day, when it's warmer, and bring them back in before nightfall. This is known as hardening off and gets them used to the big wide world outside the greenhouse.
Depending on what else you have on the greenhouse, leaving the windows and door open during the day time can work as a half step to putting them outside.
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You can also move them outside during the day, when it's warmer, and bring them back in before nightfall. This is known as hardening off and gets them used to the big wide world outside the greenhouse.