When looking up the month names, you could also use the CHOOSE function

Replace this large CASE statement
case c.m when 1 then 'January'

Simply insert the value without the id
insert into [Spielwiese].[dbo].[Table_1] (test)
values ('test');

The following 2 links are solution to the Packing Intervals Problem by Itzik Ben-Gan.
2011/04/13 ... http://blogs.solidq.com/en/sqlserver/packing-intervals/
2015/08/11 ... http://sqlmag.com/sql-server/new-solution-packing-intervals-problem

/*
Assumptions
# [EmailActivityKey] is an INT value (as it is an identity)
*/

----------------------------------------------------------
-- Option1 - Use a negative value
set identity_insert EmailActivity05082018...