If you check the Input Output Reference you will find that:

The design day input describes the parameters to effect a “design day” simulation, often used for load calculations or sizing equipment. Using the values in these fields, EnergyPlus “creates” a complete days’ worth of weather data (air temperatures, solar radiation, etc.) Normal operation uses the default range multipliers as shown in Figure though users may choose to input their own multiplier schedule. Likewise, normal operation specifies one “humidity indicating condition” which is used to calculate the humidity ratio at maximum temperature – this is used as the constant humidity ratio for the entire day. Again, this can be overridden by specifying a relative humidity schedule or requesting generation of an hourly wet-bulb temperature profile. Multiple design days may be specified.

From there since I assume you don't need any sizing (at least for HVAC system) you don't need design days.