When can I plant seeds outside?

I would like to plant zinnia seeds and other annuals outside now in Cleveland, zone 6

When can I plant seeds outside?
It should give planting suggestions for each zone on the seed packet. If it's been warm where you are for at least 3 or 4 weeks you could probably plant but it might be best to start them indoors and move them outside later.
Reply:read the back of the package.