Marriages are not conducted on Thiruvonam nakshatra day. So why Thiruvonam nakshatra day is is bad for conducting marriage?. The reason for it is found in Daivajna Sampurna Chandrika, a book on astrology.
Marriages are not to be conducted under Sravana, as it would make on give up desires and turn to vairagya (renunciation). According to Daivajna Sampurna Chandrika, a book on astrology, the star has malefic effect of the planet Mars, which would bring adverse effects on the life of the partner after marriage.
The presiding deity of Thiruvonam nakshatra is Bhagavan Vishnu (especially his Venkateswara manifestation).
It is considered an auspicious star for naming a baby, first haircut, commencement of learning, thread ceremony, commencement of medical treatment, inauguration of temples, going on pilgrimage, and such other auspicious functions.