It can be stipulated formally in a contract of employment and is considered part of the remunerations package along with Holiday allowance. The term has also since become used by people to reference taking a day off work for no normally accepted reason (such as sick, grievance or holiday) even if they have no official "Duvet day" entitlement with their employer.
It differs from Holiday allowance in that no prior notice is needed. An employee receives an allocation of days where if he or she gets up in the morning and doesn't want to go to work for any reason, he or she can use a "Duvet day".
Originally given to employees by UK public relations company Text100 in 1995, it has grown in popularity as some companies aim to address the changing work-life climate where people work longer hours.