SQL & PL/SQL

I'm trying to do this:

select ('fields')
from ('tables')
where (case when (:p_note_date='week')
then vmen.note_date between (trunc(sysdate,'day') and trunc(sysdate,'day')+6)
when (:p_note_date='month')
then vmen.note_date between (trunc(sysdate,'month') and last_day(sysdate))
else vmen.note_date between :p_start_date and :p_end_date
end)

I keep getting errors. Can't we use a 'CASE' statement in a 'WHERE' clause? I've tried using 'DECODE' and it's not working either.

Thanks!!
Kris