ok, this is the situation:
Real Madrid wants to play tomorrow at 17:00 because there is no guarantee the lighting will be fixed by nighttime tomorrow.
Rayo wants to play tomorrow at 20:00, they say that they are certain the issue will be fixed by then and that they aren't willing to play at 17:00 because their fans will be working and the stadium will be empty. Their opinion isn't valid though as they are responsable of the issue. If Real Madrid wants to play at 17:00 and the LFP understands it is convenient to play then, Rayo will have to give in.
If the LFP decided to say "fuck it, if you can't reach an agreement by yourselves we'll do things our way", the game will be played in february. This is not very likely.