date_trunct() and start of week

Hi,

while using date_trunc('week', some_date) to get the date of the first day of the week I noticed that it was working as
expected:Monday is considered the start of the week.  

I assume this depends on some locale setting, but I can't figure out which it is, so I can make sure this is not
"accidently"changed. I tried changing LC_TIME (American_America) but that still returned Monday as the first day (my
understandingis that in the States Sunday is considered the start of the week) 

Any pointers are appreciated (did I miss it in the manual?)

Regards
Thomas