Tithis are specific phases of the moon and calculating time by these phases helps us choose auspicious times for certain practices or important life events. A majority of Vedic customs are based on these specific times where the moon is more illuminated or less so.
A tithi is one full day of the Lunar month, different from a solar day and unrelated to the 24-hour day. There are thirty tithis (dates) in one lunar month. They consist of one bright tithi or full waxing moon (Shukla), one dark tithi or dark waning moon (Krishna). There are fourteen days leading to the full moon and fourteen days leading to the dark moon, for a total of thirty tithis in a month.
Each year we post these lunar dates for the year for those who incorporate them into their practice times. During Vajra Panjar teaching retreats (VP1), Dhyanyoga Centers students learn how to practice with the tithis in depth.
In certain spiritual practices, the tithi is mentioned at the beginning of a practice as a part of the process of stating where you are in time when beginning the practice.