TSQL - Date ranges

Hi Experts,

I have this data:

transactionDate
2005-07-15 16:12:46.000
2005-07-15 16:12:46.000
2005-07-15 16:19:22.000
2005-07-18 09:31:29.000
2005-07-18 09:31:29.000
2005-07-18 11:00:36.000
2005-07-18 11:00:36.000
2005-07-20 10:58:54.000
2005-07-20 14:21:49.000
2005-07-20 14:21:49.000
2005-07-20 04:04:59.000
2005-07-20 16:13:38.000
2005-07-20 16:13:38.000
2005-07-21 08:32:49.000
2005-07-21 08:32:49.000
2005-07-21 08:36:49.000
2005-07-21 08:45:13.000
2005-07-21 15:14:59.000
NULL
2005-07-22 14:03:08.000
2005-07-22 14:03:08.000
2005-07-25 09:07:57.000
2005-07-25 09:07:57.000
2005-07-25 11:43:37.000
2005-07-25 11:45:24.000
2005-07-25 11:46:52.000
2005-07-25 11:47:34.000
2005-07-25 11:48:25.000
2005-07-25 11:50:22.000
2005-07-25 12:39:06.000
2005-07-25 12:48:04.000
2005-07-25 12:48:59.000
2005-07-26 10:38:08.000
2005-07-26 10:38:08.000
2005-07-26 10:44:08.000
2005-07-26 10:44:08.000
2005-07-26 18:16:36.000
2005-07-26 18:16:36.000
2005-07-27 09:30:16.000
2005-07-29 10:51:09.000
2005-08-01 17:31:21.000

Select all Open in new window

I took the Null out so please disregard.
 
I will be inserting this into:

Table.ControlDate Where transactionDate is the first day of the month
Table.LastDate Where transactionDate is the first day of the next month
Table.PostDate Where transactionDate is the first day of the next month

So:
Table.ControlDate  = 2005-07-15 16:12:46.000
Table.LastDate = 2005-08-01 17:31:21.000
Table.PostDate = 2005-08-01 17:31:21.000

I hope this is clear and if not please let me know.

Thanks