Is it possible to set start of week for T-SQL DATEDIFF function?

Yes it possible

SET DATEFIRST 1; -- Monday

from http://msdn.microsoft.com/en-us/library/ms181598.aspx

It appears datediff doesn't respect the Datefirst, so make it do so run it like this

create table #testDates (id int identity(1,1), dateAdded datetime)
insert into #testDates values ('2009-07-09 15:41:39.510') -- thu
insert into #testDates values ('2009-07-06 15:41:39.510') -- mon
insert into #testDates values ('2009-07-05 15:41:39.510') -- sun
insert into #testDates values ('2009-07-04 15:41:39.510') -- sat

SET DATEFIRST 7 -- Sunday (Default
select * from #testdates where datediff(ww, DATEADD(dd,-@@datefirst,dateadded), DATEADD(dd,-@@datefirst,getdate())) = 0
SET DATEFIRST 1 -- Monday
select * from #testdates where datediff(ww, DATEADD(dd,-@@datefirst,dateadded), DATEADD(dd,-@@datefirst,getdate())) = 0

Stolen from

http://social.msdn.microsoft.com/Forums/en-US/transactsql/thread/8cc3493a-7ae5-4759-ab2a-e7683165320b

I have another solution. This should be easier to understand, correct me if I am wrong

SET DATEFIRST 1
select DATEDIFF(week, 0, DATEADD(day, -@@DATEFIRST, '2018-04-15 00:00:00.000'))

We subtract '-1' from date and Sunday will become Saturday (which is 7nth day of week) and Mondфy(2) will first day of week