The code to find the first Monday of the year is fairly simple:

First find the 7th day of the year (Jan 7):

dateadd(yy,datediff(yy,0,a.DT),6)and then find the Monday on or before that date:

dateadd(dd,(datediff(dd,0, JanuarySeventh )/7)*7,0)

Does not depend on any setting of language or datefirst.

select

DT,

FirstMonday =

dateadd(dd,(datediff(dd,0,dateadd(yy,datediff(yy,0,a.DT),6))/7)*7,0)

from

( -- Test Data

select DT = getdate()union all

select DT = '20111231'union all

select DT = '20121231'union all

select DT = '20131231'union all

select DT = '20141231'union all

select DT = '20110101'union all

select DT = '20080229'

) a

Results:

DT FirstMonday

------------------------ -----------------------

2011-08-19 14:42:26.310 2011-01-03 00:00:00.000

2011-12-31 00:00:00.000 2011-01-03 00:00:00.000

2012-12-31 00:00:00.000 2012-01-02 00:00:00.000

2013-12-31 00:00:00.000 2013-01-07 00:00:00.000

2014-12-31 00:00:00.000 2014-01-06 00:00:00.000

2011-01-01 00:00:00.000 2011-01-03 00:00:00.000

2008-02-29 00:00:00.000 2008-01-07 00:00:00.000