Late spring, like the 3rd/4th week of May. Here in Vermont, planting too soon can cost you with a late frost morning which we always get in May. The apple growers hate this but it's always a possibility. This late planting sucks as Vermont summers are short, 3 months max. Indoor April planting helps, it's what I do with my summer picotee begonia displays. Wi th petunia's, there's no need as they're quick growers, especially trailing petunias! But I always look forwards to May as it's time to get my hands dirty. LOL
Cheers,
Robin