Do a wait stats analysis. There are 52 parallel operations in the slow plan and only 45 parallel operations in the fast plan.  To break it out a bit more:

Just took a look at some of the joins, and yes, they could be problematic as well.  Conditional joins can be painful.

Jeff puts it well, many problems can...

Just looking at the query itself shows that it could deal with a rewrite.  The use of the YEAR function in the WHERE clause is going to cause scans on...

See if this helps, it is based on us-English settings.

DECLARE @TestDate DATETIME;

SET @TestDate = '2017-11-19';

SELECT DATEADD(WEEK,DATEDIFF(WEEK,0,@TestDate) - 2, 0), DATEADD(DAY,6,DATEADD(WEEK,DATEDIFF(WEEK,0,@TestDate) - 2, 0));
SELECT DATEADD(WEEK,DATEDIFF(WEEK,0,@TestDate) -...

Still incomplete.  I need to know the date that is input and the date (or dates) that are output, not that "I click a button and this is returned.
The...