Tithi is considered as the first phase or portion (Kala) of the 16 phases of the Moon. The 15 days, commencing from Amavasya (the last day of the dark half of a lunar month) to Purnima (Full moon), are called the Tithis of the Shukla-Paksha (brighter phase) and the days commencing from Purnima to Amavasya are called Krishna Paksha (darker phase).
The fifth part of the Panchang is Vaar or weekday. Vaar is the time between one sunrise and the next. There are seven Vaars, i.e, Ravivaar, Somvaar, Mangalvaar, Buddhvaar, Brihaspativaar, Shukravaar and Shanivaar. Mangalvaar and Shanivaar are considered unlucky for any auspicious activity. Ravivaar is partially auspicious. Somvaar, Buddhvaar, Brihaspativaar and Shukravaar are considered good for auspicious works.
Nakshatra is the first ang, or part of the Panchang. According to Indian Vedic astrology, there are 27 Nakshatras. In muhurtha astrology, a 28th Nakshatra is also counted, called the Abhijeet Nakshatra. Abhijeet Muhurtha is considered very lucky for all significant work. Hence if you include Abhijeet, the Nakshatra count becomes 28.