Vedic astrology defines twenty seven Yogas.
Following 8 Yogas are considered Ashubh i.e. inauspicious for marriage.
It is believed that marriages held during above Nitya Yogas results in the death of the bride or the bridegroom. The bride might suffer from various diseases. The bridegroom might become drunkard, meat eater and timid. The couple might suffer from the death of the son.
Hence all above Yogas should be avoided while decided auspicious marriage date. Rests of the 19 Yogas are considered auspicious for marriage.