My Note
Shraaddh can be performed if there is a nightmare or bad period also as per astrology. So as per this Tithi is least important of the death of the person to whom Shraaddh is performed. Many people ask if Shraaddh could not be performed on this particular Tithi. Varaah Puraan gives the answer here which need not require any clarification.

Appropriate Stars to Perform Shraaddh
The various Nakshatra like Anuraadhaa, Vishaakhaa and Swaati are considered to be very auspicious for performing Shraaddh - especially, if the day happens to be Amaavasyaa. Shraddh performed on such a day pacifies the ancestor's soul for eight years.

Similarly the combination of Amaavasyaa with either Pushya Nakshatra or Punarvasu Nakshatra is believed to pacify the ancestors soul for 12 years.

Some other days which are considered to be very auspicious for performing Shraaddh are - the 3rd day of the Shukla Paksh of the Lunar month Vaishaakh, the 9th day of the Shukla of the Hindu month Kaarttik, the 13th day of the Krishn Paksh of the Lunar month Bhaadrapad and Amaavasyaa falling in the Lunar month of Maagh.

A man desirous of performing Shraaddh can do so on any of the above mentioned days. He should take bath in a holy river and offer Tarpan (libation) to his ancestors.

My Note
Good stars to perform along with combination of Tithi has been preferred. When Tithi and star join the anvestor's soul get pacified for 8-12 years as per Varaah Puraan.

So why should we get upset for not able to perform on a particular month and Tithi or star Shraaddh for the deceased. Knowingly or unknowingly our Shraaddh may fall on such occasions as told.

Eligibility of Braahman to Participate and Perform in Shraaddh
Describing the types of Braahman, considered to be eligible to preside over the rituals of Shraaddh, Maarkandeya told sage Gaurmukh - "The following types of Braahman are eligible to perform Shraaddh - "Trinachiket", "Trimadhu", "Trisuparn" and the Braahman who are well versed in all the Ved. The Shraaddh rituals can be performed by any of the following relatives of the deceased, apart from his son - maternal nephew, grandson (daughter's son), father-in-law, son-in-law, maternal uncle, etc.

Braahman Not to be Invited as Shraaddh Braahman
The following types of Braahman should never be invited at the Shraaddh at the Shraaddh ceremony -
--A deceitful or a wicked Braahman, a Braahman who is a habitual backbiter or who is accused of theft, a Braahman who has married a Shoodra woman and a Braahman who earns his livelihood by working as a priest.

How to Invite Brahmins to Shraddh Ceremony
Invitation should be given to the all the Brahmins one day in advance. if an uninvited Braahman arrives to attend the Shraaddh ceremony they should be treated with respected. The Braahman who is supposed to perform the rituals should wash the feet of other Braahman to show his respect towards them. He should then help them perform Aachaman (rinsing of mouth). Finally, the invited Braahman should be fed after the rituals have been completed.

As far as the number of invited Braahman to the Shraaddh ceremony is concerned, the scriptures have strictly put it to odd numbers-one, three, five, etc Similarly, the number of invited Braahman for the Shraaddh of the deities is concerned the number has been put to even number - two, four, etc. But, if the person finds it difficult to invite the above number of Braahman then he can invite only one Braahman for both the types of Shraaddh.

In the Shraaddh ceremony related with the deities the Brahmins should always have their food facing East. On the contrary, in the Shraaddh ceremony of the ancestors the Braahman should have their food facing North.

Method of Performing Shraaddh
A person performing the Shraaddh rituals should have his seat made of Kush grass. Then he should invoke the deities and make offerings of Arghya to them. While offering Arghya to the deities water and barley are commonly used along with other articles like sandalwood, incense, etc. During the entire course of Shraaddh rituals Yagyopaveet (sacred thread) should be worn in an inverse position - across the right shoulder, and not across the left shoulder as it is normally worn, for Pitri. While performing the Shraaddh of ancestors offerings of articles like sesame seeds, water and Kush grass are normally made. It is customary to scatter sesame seeds near the place where Braahman are having food in the Shraaddh ceremony. The performer of the Shraaddh rituals should imagine that the food partaken by them are actually satiating the ancestors. The chanting of Rakshodhan Mantra is a must. One important characteristic of the rituals connected with Pind Daan is the offerings made to manes in the form of sesame seeds and water. Another important aspect is that Pind is offered on the Kush grass which are kept facing south-first in the name of one's dead father and then in the name of one's grandfather and other ancestors.

Balivaishwa Dev Ritual
The performance of Balivaishwa Dev rituals marks the end of the Shraaddh ceremony after which the person should sit down along with all the invited guests to have his food. Ancestors become satisfied if Shraaddh is performed in their names and thus all the desires of such a man are fulfilled. Three things have great importance in Shraaddh rituals - black sesame seeds, the auspicious Muhoort named 'Kutup' and the grandson (daughter' s son). Donating silver is considered to be extremely auspicious. A person performing the Shraaddh rituals should not venture out of his house till the completion of the entire ceremony.