Extract day of week from date field in PostgreSQL assuming weeks start on Monday

From the manual

isodow

    The day of the week as Monday (1) to Sunday (7)

So, you just need to subtract 1 from that result:

psql (9.6.1)
Type "help" for help.

postgres=> select extract(isodow from date '2016-12-12') - 1;
  ?column?
-----------
         0
(1 row)
postgres=>