You probably need to specify the first day of your week with set datefirst:
set datefirst 1;
select datepart(week, '2017-02-01');
returns 6
Depending on the default language, your datefirst might be set to 7.
set datefirst 7;
select datepart(week, '2017-02-01');
returns 5
rextester demo: http://rextester.com/KEPB52852