WHERE
...
AND
 (EXTRACT(MONTH FROM MY_DATE_FIELD) = EXTRACT(MONTH FROM CURRENT_DATE)) 

AND
 (EXTRACT(MONTH FROM MY_DATE_FIELD) = EXTRACT(MONTH FROM CURRENT_DATE) - 1)

EXTRACT(MONTH FROM DATEADD(-1 MONTH TO DATE '2020-01-20'))

EXTRACT(WEEK FROM MY_DATE_FIELD) = EXTRACT(WEEK FROM DATEADD(-1 WEEK TO CURRENT_DATE))

EXTRACT(WEEK FROM MY_DATE_FIELD) = EXTRACT(WEEK FROM CURRENT_DATE)

SELECT
  CURRENT_DATE - EXTRACT(WEEKDAY FROM CURRENT_DATE) + (7 - EXTRACT(WEEKDAY FROM CURRENT_DATE)) AS START_DATE_OF_WEEK,

DATEADD(DAY, (EXTRACT(WEEKDAY FROM DATE '2020-01-10') * -1) +1, DATE '2020-01-10') AS WEEKSTART,

DATEADD(DAY, (EXTRACT(WEEKDAY FROM DATE '2020-01-26') * -1) +1, DATE '2020-01-26') AS WEEKSTART3,

DATEADD(DAY, (EXTRACT(WEEKDAY FROM DATE '2020-01-05' - 1) * -1), DATE '2020-01-05') AS WEEKSTART4,

SET TERM ^ ;

CREATE OR ALTER FUNCTION GET_FIRST_DAY_OF(PERIOD STRING16, D DATE2)
RETURNS DATE2
AS
BEGIN
  IF (D IS NULL) THEN
    D = CURRENT_DATE;

  IF ((UPPER(PERIOD) = 'WEEK') OR (UPPER(PERIOD) = 'W'))  THEN
    RETURN DATEADD(DAY, (EXTRACT(WEEKDAY FROM D - 1) * -1), D);

  IF ((UPPER(PERIOD) = 'MONTH') OR (UPPER(PERIOD) = 'M'))  THEN
    RETURN D + 1 - EXTRACT(DAY FROM D);
END
^

SET TERM ; ^

SELECT
D,  
EXTRACT( WEEKDAY FROM D)          AS AMERICAN,
EXTRACT( WEEKDAY FROM D - 1) + 1  AS ISO8601FROM T;