This query is producing counts of logins per hour:

SELECT DATEADD(hour, DATEDIFF(hour, 0, EVENT_DATETIME), 0), 
       COUNT(*)
  FROM EVENTS_ALL_RPT_V1
 WHERE EVENT_NAME = 'Login' 
   AND EVENT_DATETIME >= CONVERT(DATETIME, '2010-03-17 00:00:00', 120) 
   AND EVENT_DATETIME <= CONVERT(DATETIME, '2010-03-24 00:00:00', 120)
GROUP BY DATEADD(hour, DATEDIFF(hour, 0, EVENT_DATETIME), 0)
ORDER BY DATEADD(hour, DATEDIFF(hour, 0, EVENT_DATETIME), 0)

...with lots of results like this:

Datetime                 COUNT(*)
----------------------------------
2010-03-17 12:00:00.000  135
2010-03-17 13:00:00.000  129
2010-03-17 14:00:00.000  147

What I need to figure out is how to query the average logins per hour for a given day. Any help?