The days of the nine bad omens are considered the most inauspicious days of the year. It starts on the 6th day of the 11th Tibetan month at 12:00 noon, and ends at 12:00 noon on the 7th day. At the time of the Buddha, one practitioner tried to perform a lot of positive actions on this day, and nine bad things happened to him. Consequently, Buddha proclaimed that on this date each year in the future, it is best not to try to accomplish much. Definitely it is a bad time to start projects or start a retreat. Traditionally, Tibetans play games and have picnics on this day.

The 24 hours after the nine bad omens end (12:00 noon on the 7th to 12:00 noon on the 8th) is considered the day of ten good omens. On this day, ten good things happened to the same practitioner who had experienced the nine bad omens, as he continued to persevere in his performance of positive actions. This period is considered very auspicious for starting projects, retreat, etc. However, Tibetans traditionally play games and have picnics on this day as well.

Source: Alexander Berzin on Study Buddhism.