The seasonality of the Pacific Decadal Oscillation (PDO) is examined in an analysis based on historical PDO indices and associated atmospheric fields for each calendar month. The emphasis is on the extended winter (November-March) and extended summer (May-September) seasons. The atmospheric fields of 500 mb geopotential height, surface air temperature, and sea level pressure vary not only across seasons, but also from one calendar month to another within a season, although month-to-month continuity is apparent. For example, a trough-ridge-trough tripole extending eastward over North America is consistently associated with a positive PDO index from November to February, but this teleconnection tends to break down in March. Although the variance of the atmospheric fields is greater in winter than in summer, a positive summertime PDO index is associated with an anomalous ridge that shifts eastward from June to July, bringing above-normal temperatures to the eastern United States and negative temperature anomalies to the West. However, the atmospheric pattern changes substantially in August. The strong seasonality of the atmospheric component contrasts with the much smaller seasonal variability of the sea surface temperature anomalies that define the PDO index, indicating that the atmosphere's response to the sea surface anomalies is dependent on the atmospheric base state.