Aggregate an Aggregate, Sql Server

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create table all_activities (id int identity(1,1), mydatetime datetime, Activity varchar(64))
    insert into all_activities values
     ('20170119','The door opened')
    ,('20170119','The door opened')
    ,('20170119','The door opened')
    ,('20170118','The door opened')
    ,('20170117','The door opened')
    ,('20170118','The door opened')
    ,('20170117','The door opened')
    ,('20170118','The door opened')
    ,('20170117','The door opened')
    ,('20170118','The door opened')
    ,('20170117','The door opened')
    ,('20170118','The door opened')
    ,('20170117','The door opened')
    ,('20170118','The door opened')
    ,('20170117','The door opened')
    ,('20170118','The door opened')
    ,('20170117','The door opened')
    ,('20170115','The door opened')
    ,('20170114','The door opened')
    ,('20170114','The door opened')
    ,('20170113','The door opened')
    ,('20170112','The door opened')
    ,('20170112','The door opened')
    ,('20170112','The door opened')
    ,('20170112','The door opened')
    ,('20170112','The door opened')
    ,('20170111','The door opened')
    ,('20170110','The door opened')
    ,('20170110','The door opened')
    ,('20170110','The door opened')
    ,('20170110','The door opened')
    ,('20170110','The door opened')
    ,('20170110','The door opened')
    ,('20170110','The door opened')
    ,('20170110','The door opened')
    ,('20170110','The door opened')
    ,('20170110','The door opened')
    ,('20170109','The door opened')
    ,('20170109','The door opened')
    ,('20170109','The door opened')
    ,('20170109','The door opened')
    ,('20170108','The door opened')
    ,('20170107','The door opened')
    ,('20170107','The door opened');

--Declare @X int = 2; -- at least (x) per day
    --Declare @Y varchar(7) = '2017-01'; -- at least (x) per day
    Declare @Activity varchar(64) = 'The door opened';
    Declare @DailyThreshold int = 2;
    Declare @Month varchar(7) =  '2017-01'; 
    Declare @NthDay int = 5; -- for more then 5 days in a month(Y)

;with countCte as (
      select distinct
          TheMonth= convert(varchar(7),convert(date,mydatetime),120)
        , TheDate= convert(varchar(10),convert(date,mydatetime),120)
        , TheDay = datepart(day,mydatetime)
        , Activity
        , DailyActivityCount = count(*) over (
              partition by convert(date,mydatetime), activity
              )
        , runningActivityCount = count(*) over (
              partition by convert(varchar(7),convert(date,mydatetime),120), activity 
              order by convert(date,mydatetime)
              )
        , MonthlyActivityCount = count(*) over (
              partition by convert(varchar(7),convert(date,mydatetime),120), activity
              )

from all_activities
      where activity = @Activity
        --and mydatetime >= dateadd(month, datediff(month, 0, getdate()), 0)
        --and mydatetime < dateadd(month, datediff(month, 0, getdate())+1, 0)
        and mydatetime >= dateadd(month, datediff(month, 0, '20170101'), 0)
        and mydatetime < dateadd(month, datediff(month, 0, '20170101')+1, 0)
      )
    , cte as (
        select *
          , rn = row_number() over (
              partition by Activity 
              order by TheDate
              )
          --, 
          from countCte
          where 1=1
            and DailyActivityCount > @DailyThreshold
            and TheMonth = @Month
    )
    select 
        top 1
        *
      from cte
      where rn > @NthDay
      order by TheDay

View schema
Execution time: 0,05 sec, rows selected: 12, rows affected: 44, absolute service time: 0,2 sec