The Barque of Aset from the backside of the west 1st Pylon at the Temple of Aset at Philae.

This is an effort to find out what festivals were celebrating Aset in ancient times. I have chosen to mention also some of the festivals which focused on Wesir and/or Heru, and the Ennead, as these also encompassed Aset to some degree. Either as spouse to Wesir or mother to Heru or part of the Heliopolitan Ennead, the celebrations most likely were also directed at, or included, her.

We know about the ancient festivals from calendar inscriptions on temple walls, mainly from Late Period temples like Kom Ombo, Edfu, Esna and Dendera but there are also some extant papyrii (the Lahun Arcihve) which enable us to reconstruct events.

Old Kingdom
Festivals were celebrated at least since the Old Kingdom and there are fragments of a festival calendar at the solar temple of Niuserre at Abusir. There we find mentionings of the 'Five Days over the Year', i.e. the Epagomenal Days. According to myth, these were days outside of time which Djehuty won by playing a board game with the moon. They celebrate the birthdays of the gods of the Ennead, of which Aset is one. So there´s reason to presume that here are signs of an early festival day of her. These Epagomenal Days appear in calendars from every time period thereafter.

Middle Kingdom
From the Middle Kingdom there are no calendars surviving on temple walls at all. But some papyrus fragments remaining from the Lahun archive (Fayium area) dated to the reign of Sesostris II, give enough info for a reconstruction of a festival calendar although many dates are missing. The festivals celebrated appear to be mostly the same as during the Old Kingdom, and accordingly the births of Wesir, Heru, Set, Aset and NebtHet were celebrated on the five Epagomenal Days between the fourth month of Shomu and the first month of Akhet. That is however the only place where the name of Aset is mentioned in these fragments.

5 Epagomenal Days:
Days 1-5, feasts, Births of Wesir, Heru, Set, Aset, NebtHet .

The New Kingdom:
This is where we find the most elaborate temple calendars, at several places like Karnak, Abydos, Elephantine, Medinet Habu, the Ramesseum and other places. They are from, among others, the reigns of Thutmose III, Ramesses II and Akhenaten.

The Festival Calendar of Ramesses II at Abydos is where Aset appears in accordance with the myth, as spouse of Wesir and also as his mourning widow, along with her sister NebtHet, when the Mysteries of Wesir were celebrated yearly here. She is also seen in the background as one of the Ennead. The calendar gives dates for:

1st month of Akhet, day 30
Feast of Wesir and the Ennead
2nd month of Akhet, days 5 and 16
Feast of Wesir and the Ennead
3rd month of Akhet, day 17
Great and small landing in Abydos, mourning of Aset and NebtHet.

The festival day of the actual Mysteries of Wesir begin on the 4th month of Akhet, day 18 through 30. It is also worth mentioning the ritual of 'Raising of the Djed Pillar' in which the King is assisted by Aset as he raises the Djed pillar as a sign of the revival of Wesir and which is depicted in the First Hall of Wesir at the temple of Seti I at Abydos:

2nd month of Peret day 6:
Raising the Djed-pillar for Wesir; gods mourn him

Late Period:
Now mentionings of several festivals are much easier to find but it must be remembered that the calendars and inscriptions on temples from this period are Greco-Roman so there are both similarities and differences to earlier Pharaonic calendars. These were found at Kom Ombo, Edfu, Esna and Dendera There are some evidence also at Philae, among the hymn inscriptions, but if there was a festival calendar, it has sadly gone lost. I have tried to sift out every instance where Aset is directly referred to.

On the calendar of Kom Ombo, west side, from the time of Ptolemy VI, it is mentioned:
5 Epagomenal Days:
Birth of Heru, Aset, NebtHet, together with every day of appearing at his whish. It is he who asks after the state (of affairs) corresponding to the establishing(?) of these your festivals, these benefits(?), made for you by the Son of Re, Ptolemy. May you grant him renown as far as the sun´s ryas (shine), to everyone and may you rejoice(?) at seeing him, as Hapi, when he has inundated the Twol Lands.

The Edfu Large Calendar of Heru:
Fourth month of Shomu, day 2:
Taking out in procession, Aset the Brilliant, Mother of the god, residing in Behdet. Resting in the barque-sanctuary; every (kind of) good thing is offered to her.

Five days over the year:
Birth of [Aset]. The First Feast of this god is celebrated. Going out in procession, resting in the Place of the First Feast, oppo[site] Re; invocation [....]; offering(?) is made for the purification-feast. Every instruction is carried out [....... .....] on this day. Going (in procession) from thee; resting in the Mesenet-chapel.

Another festival celebrated at this time period at Edfu, which might be mentioned, where Aset plays a part, as the Mother of Heru, is the Victory of Heru, celebrated in the 2nd month of Peret. It commemorates the battle between Heru and Set for the heritage of Wesir to the rulership of Egypt. Though the main deity is Heru, Aset is incessantly present at his side, as protectress and helper against his enemies. At the end of the festival, Aset celebrates her son´s victory at the head of two choruses of singers while Set is being slain. He is here represented by a hippopotamus made of dough, which Aset hacks to pieces and distributes to the other deities present in the temple. She directs which parts should go to the four corners of Egypt, that the foreleg should got Weisr in Busiris, the rump to NebtHet and for herslef she keeps the forepart and the hindleg. Then she directs Heru to give the bones to the cat, the fat to the worms and the suet to the 'Young Harponeers'. After this, Heru´s victory over the enemies of the land and the gods, including Wesir and Aset are celebrated and he is crowned King of Egypt .

The Edfu Calendar of HetHert:
2nd Akhet, day 6:
Festival of Aset the Great, Lady of the Two Lands. It is the beginning of writing for her (of her annals) by her mother Tefnut, as for her elder brother Wesir.
4th Peret, day 4:
[Feast?] of Pakhet; it is the Eye of Heru; feast of Heru, Lord of life. (Date) of the lunar feast of this month. Heru, son of Aset and of Wesir is born in it. (Celebration of) the birth-giving of the goddess, for Aset, mother of the god on this day until the 21st; this goddes goes around her town/domain.

3rd Shomu, day 12:
they celebrate the marriage-contract of the Mistress of mankind [............] Isis; Ihy is born then; [feast?] of Re.

Five Days Over the Year:
Birth of Aset. Feast of 'revealing the 'face' of this goddess with her ennead. Performing all the rites of the feast of robing [...] according to the ritual of the place of the First Feast. They make a great oblation of bread, beer, oxen, fowl, wine, milk, pome-granate-wine, [gazelles, oryx, ibexes (?)], cranes, pigeons, fattened ducks, with fresh vegetables and all (kinds of) fruit. It is (so) sweet to serve the Beautiful One with right offerings! This goddess is taken out in processions as a female Bes; consecrating [.........]; invocations and summons. All the rites are celebrated in accord with what is the ritual.

From the temple of Esna:
1st month of Inundation (Akhet), 10th day:
making the procession of Aset in Pi-Neter, in order to present offerings to her brother Wesir and to ...... Making the procession of the same goddess at night-time; return to the temple of Khnum.
12th day:
Making the procession of the same goddess in the temple of Pi-Sahure; returning to Esna at night-time.( as it says 'the same goddess',is this Aset?)
2nd month of Inundation (Akhet), 6th day:
Feast of Menhyt; the ladies play their tambourines for her. Feast of Aset; they call it "the beginning of making festivals" (?).
Five Epagomenal Days: 4th day:
Birth of Aset; that good feast of the sky and earth; presenting the material to Khnum and his ennead.

Dendera
At Dendera Aset and HetHert are both honored. There is a small temple to Aset on the precinct where her birth is consecrated and inside the main temple of HetHert is a chapel which is called the 'place of birth'. The Five Epagomenal Days are represented with depictions of the respective deites on the southeast columns of New Year´s Kiosk located on the roof of the temple at Dendera:
[the fourth] of the Epagomenal Days the beautiful day of the night of the child in the nest, great feast in all the land'.

Philae
The Snwt-Festival.
The hymns inscriptions on the temple walls at Philae are not in the form of a calendar as such, like the ones at Edfu and Esna. However, according to L. Zabkar, in the VI hymn there is mentioning of a festival being held to Isis called the Snwt-feast, i.e the 'Feast of the Sixth Day' which was originally celebrated at Heliopolis as the day when the Wedjat-eye had become complete and perfect by addition of its 'sixth' part. Thus the name of the festival but also because it was celebrated on the sixth day of the month in certain periods. It was connected to the celebration of the Summer Solstice as well as to the rising of Sirius which heralded the Inundation.

At various times in history the dates for the Snwt-festival varied, due to differences in the solar calendar which caused the dates of the summer solstice and of the rising of Sirius to glide apart. At the time of Amenhotep II it was celebrated on the 6th day of the third month of Shomu, but in the Ptolemaic Era, the snwt festival was celebrated at Edfu on the 7th of the third month of Shomu (23 August 237 BCE), the summer solstice on 23 June while the rising of Sirius was celebrated 17-19 July in Memphis and 12-14 July in Edfu (W. Barta, 1969).

At Heliopolis offerings was made to Re but at Philae, Aset was put in the place of Re as the one being offered to, probably in line with extolling her as the main deity here. Feasts were also apparently celebrated at Bigeh in honor of Isis which however should be separated from the regular weekly ones conducted there every tenth day in honor of Wesir.

Let´s borrow the date for the Snwt-festival from Edfu. It´s not that unlikely that celebrations were carried out at more or less the same day in these temples during this period:

3rd Month of Shomu, day 7:

From the inscriptions which are found on the south wall of the sanctuary;

Horus of the East presents to you the Great Oblation
And the Seat that is in Heliopolis is in festivity:
Offerings are made to you in Heliopolis on the Feast of the Sixth Day
Glorious are the great feasts when many festivals are celebrated for you
With rich offerings at Bigeh
Forever, for your Ka, everlastingly.

Festival of Light
Herodotus in his An Account of Egypt, mentions the Festival of Light at Sais in the Delta and describes how the whole population in the city, at a certain night, placed oil lamps around their houses and kept them burning all night long. These lamps were bowls filled with salt and oil, with the wick floating on it. Herodotus does not want reveal the 'sacred legend' behind this ritual, but says these lamps were kept burning all over the land of Egypt this night. This festival is mentioned in the Calendar of Philocalos as the 'Lychnapsia´ or the Festival of Light, celebrated on the 12th of August. In Greco-Roman times there were several festivals celebrated to Isis throughout Minor Asia and the Mediterranean countries, they were forerunners of the Catholic Candlemas.

Though Herodotos does not mention Aset in this context, during this period it was considered these lights were placed out and kept burning to help Aset on her quest to find the limbs of Wesir which had been scattered by Set.

Modern celebrations of this festival are held by The House of Netjer at the end of July.

Sources:
Dendera: Les fêtes d'Hathor - Sylvie Cauville
Religious Ritual at Abydos - Rosalie David
Isis Mére des Dieux - Francoise Dunand
An Account of Egypt - Herodotus
Temple Festival Calendars of Ancient Egypt - Sherif el-Sabban
The House of Horus at Edfu - Barbara Watterson
Isis in the Ancient World - R.E. Witt
Hymns to Isis in Her Temple at Philae - Louis Zabkar