The Liga de Fútbol Profesional (LFP) publishes every year in the middle of July, the fixtures for La Liga. From that, we just know in which week the matches take place but not the exact day and time of them, which are published around a week before the match since it depends on television agreements.
The week or weekend in which the matches take place, is hardly ever modified, but there are some cases in which it could happen.
One of the main reasons why a match could be suspended is a bad weather, which will make it impossible to play safely. Weather problems usually occur during the winter.
To postpone La Liga matches, another reason could be the overlap of matches of different competitions like the European ones which makes it more difficult to find a date for La Liga matches and usually these are the ones to be called off.
As well as these reasons, there are other less frequent motives that could cause a match to call off, but the suspension is always the last alternative to take.
If in the end a match is suspended, both the new date and time will be published several weeks before it takes place.