Margazhi month, or Margali Masam, is a highly auspicious Tamil month. It is the last month before the Uttarayana Punyakalam begins. Some Hindu communities do not hold auspicious ceremonies like marriages in the Margazhi month. This is not a general rule but is followed strictly by some Hindus.
The main reason for not conducting marriages is that people want to use the holy month to concentrate on spiritual activities.
The importance of Margali Masam was announced by Sri Krishna in the Bhagavad Gita when He said Maasanam Margasheershoham – among the twelve months, I am Margazhi.
Margazhi Month is also considered the dawn period of Devas. The belief is that the Devas who had gone to sleep during the Dakshinayana wake up in the month of Margazhi.
A human year is considered to be a day of the Devas. The day time begins with
Uttarayana Punayakalam – mid January. The night time of Devas begins during Dakshinayana Punyakalam – mid July.
The reason for not conducting marriages during Margazhi (not strictly followed by all Hindu communities) is bit philosophical - the reason is that the month is the dawn period for Devas and people do not want to engage in any other auspicious activities other than worship of Gods. They want to stay away at least in this month from worldly affairs and concentrate only on worship and religious activities like chanting of Thiruppavai.
The Sabarimala Mandala Vratham for going to Sabarimala temple also ends in this month. It is also one reason why for not conducting auspicious functions.
Interestingly, a vratam called Pavai Nombu is observed by unmarried women to get good husbands.