It depends on the current locale and what day, Sunday or Monday,
counts as start of the week. In C/POSIX, the first Sunday of the year
starts week 1:
$ LC_ALL=C ncal -wd 2015-01
January 2015
Su 4 11 18 25
Mo 5 12 19 26
Tu 6 13 20 27
We 7 14 21 28
Th 1 8 15 22 29
Fr 2 9 16 23 30
Sa 3 10 17 24 31
53 1 2 3 4
$ LC_ALL=C ncal -Mwd 2015-01
January 2015
Mo 5 12 19 26
Tu 6 13 20 27
We 7 14 21 28
Th 1 8 15 22 29
Fr 2 9 16 23 30
Sa 3 10 17 24 31
Su 4 11 18 25
1 2 3 4 5
But in de_DE.UTF-8, it's ISO 8601 (i.e. %V): weeks start on a Monday,
and week 1 must contain at least four days:
$ LC_ALL=de_DE.UTF-8 ncal -wd 2015-01
Januar 2015
Mo 5 12 19 26
Di 6 13 20 27
Mi 7 14 21 28
Do 1 8 15 22 29
Fr 2 9 16 23 30
Sa 3 10 17 24 31
So 4 11 18 25
1 2 3 4 5
$ LC_ALL=de_DE.UTF-8 ncal -wd 2016-01
Januar 2016
Mo 4 11 18 25
Di 5 12 19 26
Mi 6 13 20 27
Do 7 14 21 28
Fr 1 8 15 22 29
Sa 2 9 16 23 30
So 3 10 17 24 31
53 1 2 3 4
$ date -d 2015-01-03 +'%U %V %W'
00 01 00
$ date -d 2015-01-04 +'%U %V %W'
01 01 00
$ date -d 2015-01-05 +'%U %V %W'
01 02 01
