Syntax error: expected something between '(' and the 'current_date' keyword. - response (5) by dnoeth

When you're on TD13.10 you can use a SQL UDF, otherwise simply use the calculation:

REPLACE FUNCTION WEEK_BEGIN(cdate DATE)
RETURNS DATE
SPECIFIC week_begin_DA
RETURNS NULL ON NULL INPUT
CONTAINS SQL
DETERMINISTIC
COLLATION INVOKER
INLINE TYPE 1
RETURN 
  cdate-(((cdate - DATE '0001-01-01')) MOD 7)

This returns the begin of the week, so

SELECT WEEK_BEGIN(DATE)-1

simply subtract one: