We don't know when you will plant seeds, however if you want to know when you SHOULD plant seeds then tell us which seeds you want to plant, and also tell us where in the world you want to plant them, and I'm sure that we can give you an answer.
Seed packets should give you full instructions - when, where, how deep etc. This is pure guesswork as we don't have a clue what you wish to plant, but whatever it is you will have to protect the seed under glass, or seed trays in a greenhouse, because of the danger of frost. That's a start, anyway!