Start date and week count for first calendar week of year (PostgreSQL)

iso_weeks.sql

SELECT
  year,
  date_trunc('week', make_date(year, 1, 4))::date AS start, -- first calendar week always contains January 4th.
  extract(week from make_date(year + 1, 1, 4) - 7) AS weeks
FROM generate_series(2000, 2030) t(year);

output.txt

 year |   start    | weeks 
------+------------+-------
 2000 | 2000-01-03 |    52
 2001 | 2001-01-01 |    52
 2002 | 2001-12-31 |    52
 2003 | 2002-12-30 |    52
 2004 | 2003-12-29 |    53
 2005 | 2005-01-03 |    52
 2006 | 2006-01-02 |    52
 2007 | 2007-01-01 |    52
 2008 | 2007-12-31 |    52
 2009 | 2008-12-29 |    53
 2010 | 2010-01-04 |    52
 2011 | 2011-01-03 |    52
 2012 | 2012-01-02 |    52
 2013 | 2012-12-31 |    52
 2014 | 2013-12-30 |    52
 2015 | 2014-12-29 |    53
 2016 | 2016-01-04 |    52
 2017 | 2017-01-02 |    52
 2018 | 2018-01-01 |    52
 2019 | 2018-12-31 |    52
 2020 | 2019-12-30 |    53
 2021 | 2021-01-04 |    52
 2022 | 2022-01-03 |    52
 2023 | 2023-01-02 |    52
 2024 | 2024-01-01 |    52
 2025 | 2024-12-30 |    52
 2026 | 2025-12-29 |    53
 2027 | 2027-01-04 |    52
 2028 | 2028-01-03 |    52
 2029 | 2029-01-01 |    52
 2030 | 2029-12-31 |    52