How to retrieve the start day of the week

trunc(to_date('22-AUG-03'), 'DAY') would return '17-AUG-03'

How to retrieve the first day of the year

trunc(to_date('22-AUG-10'), 'YEAR') would return '01-JAN-10'

How to retrieve the first day of the month

Since there isn't a "first_day" function, we can use the trunc function:

trunc(to_date('22-AUG-03'), 'MONTH') would return '01-AUG-03'