Harvest:

Harvest cauliflower when heads are 6 to 8 inches (15-20 cm) in diameter but still compact and smooth. The cauliflower head is a collection of flower buds; harvest the head while it is white and before the yellow buds start to open. Cauliflower loses its flavor and texture when the buds loosen. When curds begin to loosen, separate, and open, harvest immediately. When curds fully loosen, the head is called “ricey” and is not good for eating.

For more information on growing Cauliflower, check out this great guide from Harvest To Table:
https://harvesttotable.com/how_to_grow_cauliflower/