I am a Nepalese hindu. So, Sorha Sraddha(सोह्र श्राद्ध) started yesterday. It is given a high importance in Nepal, probably also because it comes just before Dashain(Navaratri + vijaya dashami). But a few research/discussion with my relatives/friends made me feel like there was a spectrum of views about it (for instance: some were restricted themselves to take strict meals(शुद्ध खाना) while others were quite flexible.) So, I was wondering about how vedas (or any holy book) dictate about its importance and rituals expected to be done during these sixteen days. Thank you!
I wasn't sure if the Sorha Shraadh is the same as the regular Shraadh but I came across a nepali website stating the same so am going to elaborate more on the regular Shraadh rituals. The Mahabharat, Anushashan Parva, Section XCI mentions the origin of the practice of Shraadh and also gives some do's and dont's about the food items. I am sharing with you the relevant portions here:
"Bhishma said, 'Listen to me, O ruler of men (Yuddhishthira), as I tell thee how the Sraddha was introduced, the time of such introduction, the essences of the rite, and the Muni who conceived it. From the Self-born Brahman sprang Atri, O thou of Kuru's race. In Atri's race was born a Muni of the name of Dattatreya. Dattatreya got a son of the name of Nimi possessed of wealth of asceticism.
Nimi got a son named Srimat who was endued with great beauty of person. Upon the expiration of a full thousand years, Srimat, having undergone the severest austerities, succumbed to the influence of Time and departed from this world. His sire Nimi, having performed the Purificatory rites according to the ritual laid down in the ordinance, became filled with great grief, thinking continually of the loss of his son.
Thinking of that cause of sorrow the high-souled Nimi collected together various agreeable objects (of food and drink) on the fourteenth day of the moon. The next morning he rose from bed. Pained his heart was with grief, as he rose from sleep that day--he succeeded in withdrawing it from the one object upon which it had been working. His understanding succeeded in busying itself with other matters. With concentrated attention he then conceived the idea of a Sraddha.
All those articles of his own food, consisting of fruits and roots, and all those kinds of staple grains that were agreeable to him, were carefully thought of by that sage possessed of wealth of penances. On the day of the New moon he invited a number of adorable Brahmanas (to his asylum). Possessed of great wisdom, Nimi caused them to be seated on seats (of Kusa grass) and honoured them by going around their persons. Approaching seven such Brahmanas whom he had brought to his abode together, the puissant Nimi gave unto them food consisting of Syamaka rice, unmixed with salt. Towards the feet of those Brahmanas engaged in eating the food that was served unto them a number of Kusa blades was spread out on the seats they occupied, with the top ends of the blades directed towards the south. With a pure body and mind and with concentrated attention, Nimi, having placed those blades of sacred grass in the way indicated, offered cakes of rice unto his dead son, uttering his name and family.
Having done this, that foremost of Munis became filled with regret at the idea of having achieved an act that had not (to his knowledge) been laid down in any of the scriptures. Indeed, filled with regret he began to think of what he had done. 'Never done before by the Munis, alas, what have I done! How shall I (for having done an act that has not been ordained) avoid being cursed by the Brahmanas (as an introducer of strange rites)?' He then thought of the original progenitor of his race.
As soon as he was thought of, Atri endued with wealth of penances came there. Beholding him exceedingly afflicted with grief on account of the death of his son, the immortal Atri comforted him with agreeable counsels. He said unto him, 'O Muni, this rite that thou hast conceived, is a sacrifice in honour of the Pitris. Let no fear be thine, O thou that art possessed of the wealth of asceticism! The Grandsire Brahman himself, in days of old, laid it down! This rite that thou hast conceived has been ordained by the Self-born himself. Who else than the Self-born could ordain this ritual in Sraddhas? I shall presently tell thee, O son, the excellent ordinance laid down in respect of Sraddhas. Ordained by the Self-born himself, O son, do thou follow it.
Next are the instructions given by Atri Rishi to his great-grandson regarding the performance of Shraadh rituals and the paraphernalia related to it:
Having first performed the Karana on the sacred fire with the aid of Mantras, O thou that art possessed of wealth of penances, one should always pour libations next unto the deity of fire, and Soma, and Varuna.
Next come the other deities that need to be worshiped in the ritual:
Unto the Viswedevas also, who are always the companions of the Pitris, the Self-born then ordained a portion of the offerings. The Earth also, as the goddess that sustains the offerings made at Sraddhas, should then be praised under the names of Vaishnavi, Kasyapi, and the inexhaustible.
When water is being fetched for the Sraddha, the deity Varuna of great puissance should be praised. After this, both Agni and Soma should be invoked with reverence and gratified (with libations), O sinless one. Those deities that are called by the name of Pitris were created by the Self-born. Others also, highly blessed, viz., the Ushnapsa, were created by him. For all these shares have been ordained of the offerings made at Sraddhas.
By adoring all these deities at Sraddhas, the ancestors of the persons performing them become freed from all sins. The Pitris referred to above as those created by the Self-born number seven. The Viswedevas having Agni for their mouth (for it is through Agni that they feed), have been mentioned before.
Regarding the food as well there are specific instructions to exclude certain substances:
The species of paddy which should not be offered at Sraddhas are those called Kodrava, and Pulka. Assafoetida also, among articles used in cooking, should not be offered, as also onions and garlic, the produce of the Moringa pterygosperma, Bauhinia Variegata, the meat of animals slain with envenomed shafts all varieties of Sucuribita Pepo, Sucuribita lagenaria, and black salt.
The other articles that should not be offered at Sraddhas are the flesh of the domesticated hog, the meat of all animals not slaughtered at sacrifices, Nigella sativa, salt of the variety called Vid, the potherb that is called Sitapaki, all sprouts (like those of the bamboo), and also the Trapa bispinosa.
All kinds of salt should be excluded from the offerings made at Sraddhas, and also the fruits of the Eugenia Jamblana. All articles, again, upon which any one has spat or upon which tears have fallen should not be offered at Sraddhas. Among offerings made to the Pitris or with the Havya and Kavya offered to the deities, the potherb called Sudarsana (Menispermum tomentosum, Rox) should not be included. Havi mixed with this is not acceptable to Pitris.
From the place where the Sraddha is being performed, the Chandala and the Swapacha should be excluded, as also all who wear clothes steeped in yellow, and persons affected with leprosy, or one who has been excasted (for transgressions), or one who is guilty of Brahmanicide, or a Brahmana of mixed descent or one who is the relative of an excasted man. These all should be excluded by persons possessed of wisdom from the place where a Sraddha is being performed.'
Having said these words in days of old unto the Rishi Nimi of his own race, the illustrious Atri possessed of wealth of penances then went back to the Grandsire's assembly in Heaven.'"
So according to the Mahabharat, these are the rituals and food items mentioned regarding the Shraadh ceremony.