In situ, anytime after your average last frost date. Inside under lights, six weeks or so before your last frost date to have good sized plants on planting day.

I agree. I started mine 3 weeks ago under lights and they are developing lots of leaves already. They will be ready to plant outside in couple of weeks.

I live in zone 3b and I start my dwarf marigolds May 1 to set out the first week of June. By then they are a nice sized plant with buds. The larger or large flowered marigolds like Antigua or Inca get 2 weeks longer.