When does poison ivy plants come out?

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Yesterday we were out mushroom hunting and we were all over wooded areas and last night my daughter had a few spots on her. I thought it was probably just from the sun but suddenly I am feeling a bit itchy even though I don't see spots. Of course it could be cause I am in a dark room*L* I am just curious though when does it come out? I suppose I better wake her up here in a bit and check her spots. I want to get my facts first though.