The following bug has been logged online:
Bug reference: 1749
Logged by: Nick Johnson
Email address: arachnid(at)notdot(dot)net
PostgreSQL version: 8.0.3
Operating system: FreeBSD
Description: date_trunc('week', ...) is incorrect for some dates
Details:
numa=# SELECT date_trunc('week', '2002-12-31'::date);
date_trunc
------------------------
2001-12-31 00:00:00-05
(1 row)
Obviously this isn't the expected behaviour...