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 | 0.6
1996-02-01 | 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