திதி (Tithi)

In vedic timekeeping, a tithi (also spelled thithi) is a lunar day, or the time it takes for the longitudinal angle between the moon and the sun to increase by 12°. Tithis begin at varying times of day and vary in duration from approximately 19 to approximately 26 hours.

There are 30 tithis in each lunar month, named as follows:

Sl.No	Krishna paksha (dark fortnight)	Shukla Paksha (bright fortnight)	Deity and properties
1	Pratipat	Pratipat	The presiding deity of the first lunar day in Brahma and is good for all types of auspicious and religious ceremonies
2	Dvitiya	Dvitiya	Vidhatr rules this lunar day and is good for the laying of foundations for buildings and other things of a permanent nature.
3	Tritiya	Tritiya	Visnu is the lord of this day and is good for the cuttings of one's hair and nails and shaving.
4	Chaturthi	Chaturthi	Yama is lord of the 4th lunar day, which is good for the destruction of one's enemies, the removal of obstacles, and acts of combat.
5	Panchami	Panchami	The Moon rules this day, which is favourable for administering medicine, the purging of poisons, and surgery.
6	Shashti	Shashti	Karttikeya presides over this day and is favourable for coronations, meeting new friends, festivities, and enjoyment.
7	Saptami	Saptami	The 7th lunar day is ruled by Indra; one may begin a journey, buy conveyances, and deal with other such things as a movable nature.
8	Ashtami	Ashtami	The Vasus rule this day, which is good for taking up arms, building of one's defenses, and fortification.
9	Navami	Navami	The Serpent rules this day, with is suitable for killing enemies, acts of destruction, and violence.
10	Dasami	Dasami	The day is ruled by Dharma and is auspicious for acts of virtue, religious functions, spiritual practices, and other pious activities.
11	Ekadasi	Ekadasi	Rudra rules this day; fasting, devotional activities, and remembrance of the Supreme Lord are very favourable.This day has special religious significance in Hinduism and Jainism -- usually observed by fasting
12	Dvadasi	Dvadasi	The Sun rules this day, which is auspicious for religious ceremonies the lighting of the sacred fire, and the performance of one's duties.
13	Trayodasi	Trayodasi	The day is ruled by Cupid and is good for forming friendships, sensual pleasures, and festivities.
14	Chaturdashi	Chaturdashi	Kali rules this day suitable for administering poison and calling of elementals and spirits.
15	Amavasya (new moon)	Purnima (full moon)	The Vasve-devas rule the New Moon suitable for the propitiation of the Manes and performance of austerities.

A Thithi ( திதி ) is a lunar day or based on Moon's rotation around the Earth. For comparison, our Gregorial Calendar (English Calendar) is based on Sun / solar days but thithis are based on Moon/Lunar days. Tithis begin at varying times of day and vary in duration from approximately 19 to approximately 26 hours.

Basically it is the time it takes for the longitudinal angle between the moon and the sun to increase by 12°. As each thithi constitutes 12°, 30 thithis constitutes a Lunar Month. Therefore, dividing the difference in logitutes(degree) between the Sun and the Moon by 12 , will give the number of the lunar day. Thithi's duration and start time vary.

For example, 'Ekadhasi( ஏகாதசி )' is actually when the Moon is in-between 120° and 132° ahead of Sun and again in-between 300° and 312°. Hence these angles actually begin and end roundly 22 hours apart, roughly one lunar day.

First thithi is Prathamai ( பிரதமை ) and at Pournami ( பொளர்ணமி ) the Moon and the Sun are opposite to each other. Moment of new Moon or time when longitudes of the Sun and the Moon are equal, is called Amavasya ( அமாவாசை ).

Lunar Day	Names of Thithis
1st	பிரதமை [Pradhamai (or) Prathipada]
2nd	த்விதியை [Dwithiya]
3rd	த்ரிதியை [Thrithiya]
4th	சதுர்த்தி [Chathurthi]
5th	பஞ்ஜமி [Panchami]
6th	சஷ்டி [Shashti]
7th	சப்தமி [Sapthami]
8th	அஷ்டமி [Ashtami]
9th	நவமி [Navami]
10th	தசமி [Dasami]
11th	ஏகாதசி [Ekadasi]
12th	த்வாதசி [Dwadasi]
13th	த்ரயோதசி [Thrayodasi]
14th	சதுர்தசி [Chathurdasi]
15th (in bright half of the month)	பொளர்ணமி [Pournami]
15th (in dark half of the month)	அமாவாசை [Amavasya]