CREATE FUNCTION public.my_weeknum(p_date DATE)
RETURNS INTEGER
AS $body$
SELECT 
  CAST(
    CASE 
      WHEN 
        EXTRACT(day FROM p_date) <= 7 AND
        EXTRACT('week' FROM p_date) > 2
      THEN 1
      WHEN EXTRACT(ISODOW FROM date_trunc('year', p_date)) = 1
      THEN EXTRACT('week' FROM p_date)
      ELSE EXTRACT('week' FROM p_date) + 1
    END AS INTEGER) AS weeknum
$body$
LANGUAGE 'sql'
IMMUTABLE
RETURNS NULL ON NULL INPUT;

SELECT my_weeknum('2021-01-04'::DATE);