Calculate different aggregated average depending on date in table

Im pretty new to DAX/Power BI so please bear with me ... 

I want to create a column that will contain a different avearge depending on the date.  I want to compare each month to an average calculated over set periods in time. These periods might overlap and they also could change. 

I tried creating each of the averages as variables then depending on the date (if datebeween use var1 etc but i cant get it to work and im not even sure if im on the right lines. 

This is how i would do it in sql 

with main as
(select
mth
,measure
from table_name
)

,avg as

(select
avg(case when mth between to_date('01/01/2018', 'dd/mm/yyyy') and to_date('30/06/2018','dd/mm/yyyy') then measure else 0 end) jan_june
,avg(case when mth between to_date('01/05/2018','dd/mm/yyyy') and to_date('31/12/2018','dd/mm/yyyy') then measure else 0 end) may_dec
from main)

Select mth, measure
,case when mth between to_date('01/01/2018', 'dd/mm/yyyy') and to_date('31/08/2018','dd/mm/yyyy') then jan_june else may_dec end avg
,case when mth between to_date('01/01/2018', 'dd/mm/yyyy') and to_date('31/08/2018','dd/mm/yyyy') then 'jan_june' else 'may_dec' end avg

from
main
cross join avg

for this result 

MTH measure AVG AVG_1
01/12/2018 5 4.87 may_dec
01/11/2018 4 4.87 may_dec
01/10/2018 3 4.87 may_dec
01/09/2018 3 4.87 may_dec
01/08/2018 4 6.16 jan_june
01/07/2018 6 6.16 jan_june
01/06/2018 7 6.16 jan_june
01/05/2018 7 6.16 jan_june
01/04/2018 8 6.16 jan_june
01/03/2018 5 6.16 jan_june
01/02/2018 5 6.16 jan_june
01/01/2018 5 6.16 jan_june