MySQL平日/週末カウント-パートII

SELECT 
  fname, 
  MONTH(eventDate), 
  SUM(IF(WEEKDAY(eventDate) < 5,1,0)) AS WeekdayCount,
  SUM(IF(WEEKDAY(eventDate) >= 5,1,0)) AS WeekendCount
FROM eventcal AS e
LEFT JOIN users AS u ON e.primary = u.username
GROUP BY fname, MONTH(eventDate);

SELECTで集計（この場合はSUM）を実行し、それらを合計する方法（fname、MONTH）でGROUPBYを実行します。