Getting the week of a date

Dear Friends,

Postgres 7.3.4 on RH Linux7.2.

While this works for month and why not for week

test=# select date_trunc('month',current_date + ('5 month')::INTERVAL);
date_trunc
---------------------
2004-07-01 00:00:00
(1 row)

test=# select date_trunc('week',current_date + ('5 month')::INTERVAL);
ERROR: TIMESTAMP units 'week' not supported
test=#

Any idea on how to find the 3 rd Wednesday of any given month.

Thanks
Kumar