Two nimishas make one Triti.
Two Tritis make one lava.
Two lavas are one Kshana.
Ten Kshanas are one Kashta.
Thirty Kashtas make one kala.
Thirty kalas make one Muhurtha.
Thirty Muhurthas are a day and a night(24 hours).
The merging of Sun and Moon is called Amavasya. When they are furthest apart it is called Poornima.
That thithi’s which are spread from a sun rise to next day sun rise does not have relation with the next thithi is called shuddha thithi else its called khanda thiti’s.
The description of Prathama thithi
Sukhla Paksha : Prathama which has Vidha with amavasya and Shukla Prathama with Poorva vidha is better. But in the Aparahna , Prathama relation should be there. If this is not available , Prathama relation should be available in Sayahna, This type of Shukla Prathama is suitable for fasting.
Krishna Paksha that Prathama which has Vidha with Dwitheeya are suitable for fasting.
The day time is divided in to five parts called
thithi in which sun set takes place is very suitable for dhana (charity), Parayana (reading scriptures) and Upavasa (fasting).
thithi should not be less than three muhurthas but can be spread more than three muhurthas such type of thithi only is suitable for rituals.
When one fast according to thithi/nakshatra – at the end of the thithi and Nakshatra we should do Parana (eat/drink something i.e break our fast ) If we do not that that Vrutha is not complete. This is applicable to those thithis which have veda for Shudha thithis this would not happen. For Shivaratri / Shravana the Parana is at the end of the thithi and not Nakshatra (Shravana in this case)
Where Parana is prescribed at the end of thithi/nakshatra and they are beyond three Yamas at night, the Parana can be performed in the morning after we get up. If we are not able to observe in the Poorva Veda thithi , fast can be performed in Uthara Veda thithi also. Depending on the kala (duration) prescribed for the Vrutha , it has to be observed depending on what thithi pervades during that time.
On Sunday, Amavasya and Pournami, we should not take food at night.
In case of Chaturdasi and Ashtami , we should not take food during day time.
In the Parva , food should not be taken at night.
In case of eclipse, after eclipse is over, only you can take food.
In the night if there is an eclipse, we should not take food at night.
Ganesha vrata should be observed on chathurthi mixed with thrithya but Chathurthi should be prevalent in the after noon . If it is not there but if there is Chathurthi in the madyahna of the next day, leave out Chathurthi with thrithya and observe it on the next day .
Even though generally shasti thithi is Para vidha and is good for fasting but in case of Skanda sashti , Poorva Vidha is considered as superior [If the relation is with in the first three muhurthas (6 nazhikas-144 minutes)] Vasistha says that in case of Krishnashtami, Skanda Sashti , Shiva Rathri and Chathurthi , Poorva Vidha is appropriate for observation.
Shukla Paksha Ashtami and Shukla Chaturdasi are more suitable as Para Vidha [relation with three muhurthas prior to sun set in the next day (6 nazhikas-144 minutes)]. But the Ashtami in the Krishna paksha is observed in Poorva vidha.
If in the Chitra month Shukla paksha Navami is with Punarvasu star, that is a very holy time. If Punarvasu is not there during that time , Rama Navami should be observed in the Shudha Navami during Madyahna. Here also fast should be observed. The holy time of Sri Rama Navami is superior to even one crores of solar eclipses. So any ritual done aimed at Rama , if done with devotion, would give great strength. Worship of Rama, reading the story of Rama’s birth, fast, waking up at night , and Tharpana aimed at the manes could be done. All good things including getting of salvation will then happen. This also is a nithya ritual and should definitely be observed. If not observed it will lead to sin.
Dasami thithi spreading in sun rise time in the Shukla Paksha is very holy. Dasami at Sun set in Krishna Paksha is holy. Lord Narasimha was born on a Chaturdasi at dusk. So the Vaisaka Shukla Chaturdasi which extends to night is superior. In the Shukla Chaturdasi of Sravana month Anantha Vrutha is being observed. That Chaturdasi should have connection with sun rise. So The Anantha Chaturdasi which is related to Pournami (full moon) is superior.
Determination of Shiva Rathri
The night of the Krishna paksha Chaturdasi of the Magha month is called Shiva rathri. This is liked by Lord Shiva. That is why it is called Shiva Rathri. On that night Lord Shiva himself is fully present in all the Shiva lingas. In permanently fixed Lingas as well as in those which are portable , even without Avahana, Lord Shiva is present on that day. So the night of Magha Krishna Paksha Shiva Rathri is called Shiva Priya Rathri : Skanda
Sivarathri is mentioned in Kamika Sasthra.Magha Krishna paksha night is called Shiva Rathri. Any good deed done during this day would give easy benefits. If on Magha Krishna Paksha , if Chaturdasi is fully there, without doubt Shiva Rathri should be observed on that day .If partial thithi comes Thryodasi is not desirable but Chaturdasi is. If Chaturdasi is there after sun set, it is considered as propitious.
The shiva Rathri time is determined here. Chaturdasi should be there at night. The time between 15th and 16th Nazhigai is defined as night. Chaturdasi should be there at that time. If during that time for two consecutive days Chaturdasi is there , then that day when it extends up to Pradosha is taken as Shiva Rathri. It is impossible for this to happen on both days.
On Shiva Rathri day, fast and spending awake the day and night as well as special poojas on Linga during the four Yamas should be done. The people who observe Shiva Rathri this way, would lead a life without any wants and reach the heaven of Lord Shiva. So Shiva Rathri is also classified as Nithya Kamyam.- NagaraKhanda