Sowing your seeds indoors at the proper date is critical. Counting backwards from the last possible time of frost in your area, calculate your seed starting time. Once plants reach a certain size, they need to move outside, and the climate needs to be right. If you start your seeds too soon, and your plants need to be transplanted but cannot go outside yet because the weather is too cold, they will be weak and spindly. The seed packet tells you how many days it takes for a seed to germinate, and to harvest.
You need to make a timing calculation plan for each vegetable you plan to grow.
We have picked the cucumber as an example to show you how a typical calculation plan works:
The days to germination according to the seed packet are 5 to 7 days.
The days to maturity are 58 days.
Therefore 7 days + 58 days = 65 days needed from seeding to harvest.
Your average last frost date is 1st May (check and adjust for your location).
The seed packet indicates at what stages the seedlings are ready to transplant.
With a calendar at hand you can conclude that you need to start seeding indoors around 15th April, so your seedlings can be transplanted outside around 7th May (leaving an extra week as a safety window against frost) and harvested around 20th June.
Read the seed packet!
All the information you need is printed on the back of the seed packet: how deep to sow, if a seed must be pre-soaked, spacing, light requirements, days to germination, days to maturity. Be sure to pick a variety where the number of days to maturity does not exceed the length of your growing season, as determined by the last frost date in the spring to first frost date in autumn.