exemplo de trabalhar com dias da semana

exemplo.sql

sebastian=# CREATE TABLE notas (id serial primary key, ano smallint, semana smallint, nota numeric(5,2));
CREATE TABLE
sebastian=# INSERT into notas (ano, semana, nota) VALUES(2016, 2, 83.7);
INSERT 0 1
sebastian=# SELECT
sebastian-#     (ano || '-01-01')::DATE + (semana ||' weeks')::INTERVAL AS data_inicial,
sebastian-#     nota
sebastian-# FROM notas;
    data_inicial     | nota
---------------------+-------
 2016-01-15 00:00:00 | 83.70
(1 row)

sebastian=# INSERT into notas (ano, semana, nota) VALUES(2016, 45, 0.7);
INSERT 0 1
sebastian=# SELECT
    (ano || '-01-01')::DATE + (semana ||' weeks')::INTERVAL AS data_inicial,
    nota
FROM notas;
    data_inicial     | nota
---------------------+-------
 2016-01-15 00:00:00 | 83.70
 2016-11-11 00:00:00 |  0.70
(2 rows)