There is no question that the most popular time of year to visit the Valley is in winter. This is partly because the rest of the country is experiencing it's worst weather of the year and in contrast, Phoenix seems like summer. So accept that any time after September and before May is probably best.
RSV: RSV in clinical medicine is the abbreviation for respiratory syncytial virus. See: Respiratory syncytial virus.
In some babies, however, RSV can cause serious illness. 1  Most babies who suffer serious complications from RSV are less than 6 months old. 2  RSV is also the most common cause of bronchiolitis (lung infection) and pneumonia in babies less than 12 months old.