Hardening Off Seedlings
Gradually acclimating seedlings to outdoor conditions.
Seedlings need time to acclimate to outdoor conditions (temperature, sunlight, wind) after getting their start indoors. Acclimating them to the outdoors is called hardening off.
When to Harden Off Seedlings
I start hardening off my seedlings when outdoor temps are above 50°F. This is typically in mid-late April where I live (Zone 6).