According to Hindu Panchang Tithi is the distance between the Sun and the Moon. The Sun and the Moon come together on the New moon. Then the Moon goes apart from the Sun. According to the Tamil calendar, there are thirty tithis to one month. Of these tamil tithis, one half belong to the Krishna paksham (the waning moon phase) and the other half belong to the Sukla paksham (the waxing moon phase of tithi).
It is believed that each of these tamil tithis are good for certain activity and not good for certain other activity.
For example, the first day or the new moon day is called the Amavasai tithi. A good day for offering tharpanam for the pithrus.
The day after the new moon or full moon is called the Prathama tithi. It is the day attributed to the Lord Agni. It is a good day to conduct poojas and other religious activities.
Dwitiyai tithi is the day attributed to Lord Brahma. It is a good tithi for building and construction related activities. A good day to lay foundation for a new project, for example.
Ekadasi tithi, according to Hindu Panchang, is the day of Rudran, a good tithi for praying to the Absolute and for deep contemplation and meditation.