This is an spanish language issue, sorry if this is not the proper place.

When you write in a cell something with format [weekday name] [day number], a
date is recognized by OOo, but this doesn't happens in Spanish translation when
the [weekday name] is "martes" (which stands for Tuesday)

An example:
writing "lunes 10" will show you "10/07/06" (spanish date format is DD/MM/YY),
writing "martes 11" will leave it as text.
writing "miercoles 12" will show you "12/07/06"
....

It also doesn't tries to match [weekday name], so if you write "domingo 10" you
will also get "10/07/06", but that is being discussed in another issue...