As funções da janela são suas amigas:
SELECT week_num,
min(d) AS start_date,
max(d) AS end_date
FROM (SELECT d,
count(*) FILTER (WHERE new_week) OVER (ORDER BY d) AS week_num
FROM (SELECT DATE '2018-09-05' + i AS d,
extract(dow FROM DATE '2018-09-05'
+ lag(i) OVER (ORDER BY i)
) = 1 AS new_week
FROM generate_series(0, DATE '2018-09-27' - DATE '2018-09-05') AS i
) AS week_days
) AS weeks
GROUP BY week_num
ORDER BY week_num;
week_num | start_date | end_date
----------+------------+------------
0 | 2018-09-05 | 2018-09-09
1 | 2018-09-10 | 2018-09-16
2 | 2018-09-17 | 2018-09-23
3 | 2018-09-24 | 2018-09-27
(4 rows)
