How do I find my frost dates for planting?

Heirloom suggests them for you from your farm’s location, so you start from your own place instead of a guess. When you set up your farm, enter your address, then click Auto-Fill at the frost dates step: you get your last spring frosts and first fall frosts, light and hard.

A light frost is around 0 °C (32 °F) and kills tender plants. A hard frost is around -2 °C (28 °F) and damages most crops. Check the dates against what you know of your land, and correct them if needed.

Your Game Plan then shades the frost weeks in blue, so you see when a planting risks the cold. Change the dates any time in Settings > Season & climate. The steps are in Set up your farm.