Krishna paksha is observed for 15 days in the Hindu mythology. It is considered inauspicious due to the positioning of the moon as people believe that plcement of the moon has a huge impact on the lives of people.
It is the phase of the moon which determine the Krishna paksha. Generally, it is categorized as an inauspicious. So Hindu month is generally divided into Krishna paksha(inauspicious) and Shukla paksha(auspicious). Our older relatives and minded people always advice us that we should not do any new or good work at Krishna paksha.