NEVER EVER use a selector like DATE(datecolumns) = '2012-12-24' - it is a performance killer:
DATE() for all rows, including those, that don't matchIt is much faster to use
SELECT * FROM tablename
WHERE columname BETWEEN '2012-12-25 00:00:00' AND '2012-12-25 23:59:59'
as this will allow index use without calculation.
EDIT
As pointed out by Used_By_Already, in the time since the inital answer in 2012, there have emerged versions of MySQL, where using '23:59:59' as a day end is no longer safe. An updated version should read
SELECT * FROM tablename
WHERE columname >='2012-12-25 00:00:00'
AND columname <'2012-12-26 00:00:00'
The gist of the answer, i.e. the avoidance of a selector on a calculated expression, of course still stands.