Best conditions for stucco are damp/humid (not wet), overcast days with temperatures in the 60s.

If it's dry, hot and sunny, it'll lose moisture and won't cure properly and crack a lot.

If it's rainy while the stucco is still soft, it'll ruin the finish (but if it's rainy after a few hours of placement, it's beneficial because it helps curing).

If it's freezing, it won't cure and may never cure.