sorting months according to fiscal year

Hi
I have to rearrange the months according to the fiscal year i.e from April
to march and use the same in the order by clause of a query.
I have written the following postgresql function for the same, but to_number
is returning an error.
Can you please tell me where i'm going wrong?
Instead of the function to_number can you suggest any other function that
will convert a particular month to its corresponding month number(ex:
april=4 or jan=1)

CREATE FUNCTION sort_month(to_number('April','MM')) RETURNS numeric AS $$
DECLARE

BEGIN

CASE WHEN 4 THEN 1
WHEN 5 THEN 2
WHEN 6 THEN 3
WHEN 7 THEN 4
WHEN 8 THEN 5
WHEN 9 THEN 6
WHEN 10 THEN 7
WHEN 11 THEN 8
WHEN 12 THEN 9
WHEN 1 THEN 10
WHEN 2 THEN 11
WHEN 3 THEN 12
ELSE 0
END;

$$ LANGUAGE plpgsql;

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