Like the Compitalia, the feriae Sementinae began as a rural festival, the feriae conceptivae. In the countryside they became the Paganalia where neighbors of rural districts (pagus/pagi) held a spring sowing festival. In the City, dependent on produce from far distant wheat fields, the feriae Sementina made a special sacrifice of a pregnant sow to appease Tellus and Ceres.
When the seed has been sown and the land is productive. You bullocks, crowned with garlands, stand at the full trough, Your labour will return with the warmth of spring. Let the farmer hang the toil-worn plough on its post: The wintry earth dreaded its every wound. Steward, let the soil rest when the sowing is done, And let the men who worked the soil rest too. Let the village keep festival: farmers, purify the village, And offer the yearly cakes on the village hearths. Propitiate Tellus and Ceres, the mothers of the crops, With their own kernels, and a pregnant sow's entrails. Ceres and Terra fulfill a common function: One supplies the chance to bear, the other the soil. ~ P. Ovidius Naso, Fasti, 1.662-674
We get some idea of the earlier sowing festivals first with Varro:
"The elevated soil that lies between two furrows is called the 'porca,' as though it were a sacrificial pig, and seeds were 'sacrificed' to these wheat fields, just as they said 'to sacrifice' when they gave the entrails of sacrificial victims to the Gods." ~ Varro Rerum Rustica 1.29
In Praeneste, in the Sabine territories, and among the Samnites, two special priests, called Semones, performed the sowing ritual. At Praeneste the Semones were related to the semi-divine twin Depidii shepherds who raised the hero-founder Caeculus. Caeculus, like Servius Tullius was said to have been conceived from a spark of Volcanus as his mother sat by the hearth. She became pregnant, so the story went, when a spark fell down into her bosum. His mother exposed him by a sacred spring at the Temple of Jupiter. Two girls who were sent to fetch water found the baby Caeculus. They brought him to the Depidii brothers. As in the legends of Romulus and Remus, Caeculus gathered a band of men around him who engaged in banditry. Caeculus invited the people of a nearby village to a festival and asked that they join him to found a new city. When they ridiculed his claim to divine parentage, he called upon his Father for a sign and Vulcanus sent a ring of fire around the assembled men. Among the Sabines Semo Santcus, the "Holy Sower," was identified as a young version of Jupiter or as a son of Jupiter instead. At Rome He became Sancus Fidius and was later identified with Hercules.
At times the Semones were identified as Lares paganales, protective spirits of the pagus fields. Pliny said of Tutilina that it was "forbidden by the rules of our religion to even name (Her) beneath a roof (H.N. 18.52)." The same was true of Semo Sanctus as it was for Seia and Seius, Tutilina and Tutanus, and Segesta. Rustic deities related to sowing, growing, and gathering grain remained out in the fields. Even when Semo Sanctus was taken at Rome to be a God of Oaths, His temple was required to have a hole in the roof so that oaths would only be given beneath an open sky.
From what Varro says of the seed being broadcast as a sort of offering made to the fields, we can imagine the Semones priests offering a sacrifice, probably that of a pregnant sow, and then casting the seeds before the sacrifice is fully extended over the altar fire. We find something similar at Vinalia where the flamen Dialis first orders the sacrifice made, then makes the symbolic first harvest of a bunch of grapes, also offered in sacrifice to Jupiter, before the viscera of the sacrificial animal was stretched over the altar.
In making the sacrifices, we have in Servius Honorus a quote from Fabius Pictor on "the Gods that were enumerated as the flamen Ceralis invoked them, while making sacrifice to Telluri and Cereri, are `Veruactorem, Reparatorem, Imporcitorem, Insitorem, Obaratorem, Occatorem, Sarritorem, Subruncinatorem, Messorem, Conuectorem, Conditorem, Promitorem." – Serv., ad Georg. 1, 21.
In sowing seed, whatever seed was being sown, we can also imagine the Semones praying in this manner:
Hoc rapum mihi vico sereo.
"I sow this rape seed for myself and for my neighbors." ~ Plinius Secunda, Historia Naturalis 24.116:
At Rome "fraters Arvales was the name given to those who perform public rites to the end that the ploughlands may bear fruits (Varro, Lingua Latinae 5.85)." This reference to fraters Arvales should not be confused with the Augustan sodalitas of the same name. The Augustan Arvales had nothing to do with blessing wheat fields, or ambarvalia, or with any of the other things claimed of them. They were instead an imperial cultus, an antiquarian invention, intended solely to secure the health of the emperor. In addition to Varro, who died two years before the Augustan Arvales were invented, there is a reference in the 8th century manuscript of Festus by Paulus, where it refers to Romulus and Remus as the two brothers performing rites similar to those of the Semones. In the Augustan Age other myths were invented to link Romulus to the Augustan fraters Arvales; one such myth is found in Pliny. So it becomes uncertain whether Sermones were ever at Rome, but since it was a practice that can be traced to Sabines, it was likely introduced to Rome following the Sabine War, and then projected back in time to Titus Tatius and Romulus, and later still interpreted as originating with Romulus and Remus.
"The feriae Sementivae (Seed-time festival) is that day set by the pontifices; it was named from 'sementis' (seeding), because it is entered upon for the sake of the sowing." ~ Varro, Lingua Latinae 6.26
That day is set by the priests, Why are you looking for moveable days in the calendar? Though the day of the feast is uncertain, its time is known ~ P. Ovidius Naso, Fasti, 1.659-661
The feriae Sementivae, or Sementinae, were conducted on two dates, seven days apart, for two early sowings. The dates were not fixed in the calendar, but set by the pontifices in accordance to the weather. The weather forecasts were determined by the rustic sidereal calendar that looked at the rising and setting of certain stars. Here the particular star that would have been associated with the commencement of the feriae Sementina is Rigel.
"On the eight day before the calends of February the star that Tubero calls Regia Stella sets in the morningin the breast of Leo." ~ Plinius Secunda, Historia Naturalis 18.64
In the time of Ovid and Plinius, Rigel set just before dawn on what was the night of 24/25. The date would be somewhat different today because of the progression of the equinoxes and also because the Romans used the Julian calendar rather than the Gregorian that we use in our civil reckonings today. The Julian calendar currently being 13 days behind the Gregorian means that 24/25 January by the Julian won't arrive until the night of 6/7 February on the Gregorian calendar. And the setting of Rigel just before dawn should arrive, by my crude calculations, on the morning of 9 February. This year the Pontifices have set the feriae Sementinae by the Julian calendar so that Nova Roma shall celebrate them on 7 February.