EXTERN JOIN with WHEN query

Hi all;
This is a query that I guess is not very dificult, but I'm a newbie;
I've got a lot of tables, each of them with two columns:

SELECT * FROM precal; -> (date)     (real)  fecha    | precipitacion
------------+---------------1996-01-01 |             0.61996-02-01 |             0.7
...


But in this table there are some inexistents records (some missing days)
And I would like to create lists with a full list of dates and corresponding 
precipitation data, with gaps when the row didn't exist.
So; I've created a table with a complete series of dates from 1950 up to 
date, and made the query:
SELECT fechas.fecha,precipitacion FROM fechas LEFT OUTER JOIN precal41 ON 
(fechas.fecha  = precal41.fecha);

This is perfect. But to make it better,  would like to include just the dates 
from the first one in the precal table. So, I've tried:

SELECT fechas.fecha,precipitacion FROM fechas LEFT OUTER JOIN precal41 ON 
(fechas.fecha  = precal41.fecha) WHEN fechas.fecha >= min(precal41.fecha);
With the answer:

ERROR:  parser: parse error at or near "WHEN"

Could you help me with this query?

Thanks and regards

Javier