Time Seed Starting Around Your Frost Date
Count backwards from a local last-frost average, and treat the date as a probability, not a promise.
Seed packets give a head-start figure in weeks before the last expected frost. Counting backwards from a local frost date converts that into a sowing week and a transplant week you can put in a calendar.
Frost dates are averages
A published last-frost date is a historical median, meaning roughly half of years see frost after it. Elevation, proximity to water, and urban warmth shift it within a single region.
Do not rush the transplant
Seedlings started too early become leggy and root-bound while they wait. Starting a week late is far easier to recover from than starting three weeks early.
Harden off, and keep cover ready
Gradual exposure over a week or so prepares seedlings for outdoor conditions. Keep fleece or cloches available for a late cold snap after transplanting.