If your dates column does not contain time information, you could get away with:
WHERE dates BETWEEN '20121211' and '20121213'
However, given your dates column is actually datetime, you want this
WHERE dates >= '20121211'
AND dates < '20121214' -- i.e. 00:00 of the next day
Another option for SQL Server 2008 onwards that retains SARGability (ability to use index for good performance) is:
WHERE CAST(dates as date) BETWEEN '20121211' and '20121213'
Note: always use ISO-8601 format YYYYMMDD with SQL Server for unambiguous date literals.