You can specify the whole day by doing a range, like so:
WHERE bk_date >= TO_DATE('2012-03-18', 'YYYY-MM-DD')
AND bk_date < TO_DATE('2012-03-19', 'YYYY-MM-DD')
More simply you can use TRUNC:
WHERE TRUNC(bk_date) = TO_DATE('2012-03-18', 'YYYY-MM-DD')
TRUNC without parameter removes hours, minutes and seconds from a DATE.