You can start using a blanket in your babies crib when the baby is 1 year old or 12 months old.
Never place a blanket or other soft objects in the babies crib when they are younger than 12 months old because it can pose a suffocation risk.
The suffocation risk then leads to SIDS or sudden infant death syndrome so always wait until the baby is at least 1 year old before using blankets or other soft objects in the crib.
The reason you need to wait until the child is at least 1 year old is because before 1 year old a baby has not developed the proper motor skills to be able to always turn themselves back over should they roll over on the blanket and get there mouth covered by the blanket or soft object.
So once the baby reaches 1 year old you can use a blanket in the crib but never before that.