Design day in HVAC is a term used to describe the worst-case scenario for a building’s heating and cooling needs. It is based on the hottest and coldest days of the year, and is used to determine the size and capacity of the HVAC system needed to meet the building’s needs. Design day is also used to determine the efficiency of the system, as well as the amount of energy needed to operate it. Design day is an important factor in the design of any HVAC system, as it ensures the system is able to meet the building’s needs in the most extreme conditions.