Multiple Month: Cross Join Months and Users

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https://www.codecademy.com/paths/analyze-data-with-sql/tracks/analyze-data-sql-analyze-real-data/modules/analyze-data-sql-user-churn/lessons/churn-rates-les/exercises/multi-month-ii

Hi, I am working on SQL and I don’t understand how the below statement does not work with ‘WITH’ and why are we not using ‘WITH’. If somebody can explain the difference, that would be great.

cross_join as(
SELECT *
from subscriptions
CROSS JOIN months

WITH months AS
(SELECT
  '2017-01-01' as first_day,
  '2017-01-31' as last_day
UNION
SELECT
  '2017-02-01' as first_day,
  '2017-02-28' as last_day
UNION
SELECT
  '2017-03-01' as first_day,
  '2017-03-31' as last_day
),

cross_join as(
  SELECT *
  from subscriptions
  CROSS JOIN months
)

SELECT *
FROM cross_join 
LIMIT 100;