The Slavs believed that the mirrors are portals in the world of the dead, but because other forces can penetrate through them to the human world. That is why in the day of the funeral all the reflective surfaces in the house still be covered with dark matter.

From the myths of many peoples you can see that the mirrors don't simply reflect the real thing, and retain recorded images that absorb the energy. The older the mirror, the more it sees events. But if before it was something bad, then it can remember, and therefore considered more dangerous.

So why not look in the mirror at night? The Slavs believed that if you look at your reflection in the dark, you can see the evil spirits that are on the other side of the glass. They are supplied with energy looking and unable to get out of the underworld.

In the Christmas day Slavic girls usually wondered at her spouse, it is often used for this mirror and candles. Such methods are used only the most desperate girl because of superstitions the most to resort to such ways to see their fate feared. It was believed that at this time in the mirror in the dark in the candlelight one could see not only the future but the devil himself. It is possible to be so scared of what he saw, that, at best, become a stutterer, and at worst, even developing a severe disease and die.

Answering the question, why not look in the mirror in the night, it should be noted that most of these fears are prejudice, which should be treated with humor. After all, if you believe in every legend and superstition, it can in fact be self-fulfilling disaster, as we know, thoughts are material.