Osiris's death and resurrection was associated with the full moon. A festival was celebrated for Osiris "coming to the moon" in the spring where his resurrection in the moon finished on the 15th day. The Egyptians also had a celebration for the birth of Horus in the Spring according to Plutarch.
Dionysus also had a rebirth festival celebrated in the Spring.
On the Primeval Ocean by Mark J. Smith:
Plutarch, Moralia 368C:The full moon is connected with the body of Osiris, which was dismembered into fourteen parts by Seth and subsequently made whole again. Osiris may be said to enter the sound eye in two distinct but related senses. According to the Egyptian vien, on each of the fourteen successive days during the period of the moon's waxing, a different divinity was thought to merge with that celestial body and restore one of it's missing parts...The work of these deities is completed by Thoth on the fifteenth day of the lunar month. At this time, restored to a state of wholeness through their actions, Osiris may be said to enter the sound eye.
In astronomical terms, full moon day is when the sun and the moon are in opposition. The Egyptians referred to this as the 'union of the two bulls'. They believed that, on the day in question, the rays of the two celestial bodies mingled together. More specifically, the greater light of the sun was thought to enter and illuminate the moon. Thus, texts speak of the latter as receiving the former or being supplied with it's rays. The moon is said to be illuminated by the sun, or the rays of the latter to pervade the former.
This belief is given visual expression in Egyptian paintings and reliefs which show the disk of the full moon with the right, or solar eye, representing the sun's light, inside it. In the Later Period, Osiris was frequently identified with the sun as well as with the moon. There is a marked emphasis upon this solar aspect in the texts and representations concerned with his entry into the sound eye when the moon is full. It is a solar deity, therefore, as well as in his capacity as a resurrected god, that Osiris can be said to enter the eye on the fifteenth day of the lunar month.
Plutarch, Moralia 377CMoreover, at the time of the new moon in the month of Phamenoth they celebrate a festival to which they give the name of "Osiris's coming to the Moon," and this marks the beginning of the spring.
The Dendera zodiacs as narratives of the myth of Osiris, Isis, and the child Horus by Gyula Priskin:and also those who say that Osiris is being buried at the time when the grain is sown and covered in the earth and that he
comes to life and reappears when plants begin to sprout. For this reason also it is said that Isis, when she perceived that she was
pregnant, put upon herself an amulet346 on the sixth day of the month Phaophi; and about the time of the winter solstice she gave birth
to Harpocrates, imperfect and premature,347 amid the early flowers and shoots. For this reason they bring to him as an offering the
first-fruits of growing lentils, and the days of his birth they celebrate after the spring equinox.
Dionysus:According to the Horus myth of Edfu, his birthday was on IV Peret 28,102 which, it should be remembered, by
rough calculation and within the timeline of the story fell around the time of the spring equinox...
the ancient Egyptian testimonies indicate that Horus was born on the day when the waxing crescent of the moon was first seen after the spring equinox. This scenario ‒ Horus being conceived on the day of invisibility after the autumnal equinox and being born
on the first-crescent day after the vernal equinox ‒ is in harmony with the already cited statement about the moon at Karnak that associated these lunar phases with conception and birth, respectively.
Thus the display of the houses of the lesser planets corresponds to the use of planetary exaltations in the round zodiac where
these latter evoke Horus’s birth at the spring equinox.
The Many-Headed Muse: Tradition and Innovation in Late Classical Greek Lyric Poetry by Pauline A. LeVen:
Inter-Actions: Relationships of Religion and Drama by Nelvin VosAs Bacchylides and Plutarch attest, dithyrambs were sung at
Delphi in honor of Dionysus only during the winter months, when
Apollo visits the Hyperboreans. The song subverts this traditional
association by linking from the outset Dionysus Dithyrambus not
with winter but with spring … the poet might be referring to the
problematic double birth of Dionysus Di-thyrambus
Philodamus of Scarphea, Delphic Paean to Dionysus(4th cen. BCE):The dithyramb represented more than a chant to drive the
sacred animal. The song also celebrated the new birth of the god,
Dionysus. However, Dionysus was not represented as an infant, but
as a young man. Therefore, the emphasis is as the etymology of the
word, Dithyrambos, indicates, on the second birth of Dionysus, his
adoption by Zeus. He was twice-born, once of his mother, like all
men, once of his father’s thigh, like no man. This aspect of the
dithyramb is shown clearly in a paean to Dionysus discovered at
Delphi. … Therefore, one can see that the birth of Dionysus and
the coming of spring are intimately connected. As Jane Harrison
summarizes: “The Dithyramb … is not only a song of human
rebirth: it is the song of the rebirth of all nature, all living things; it is
a Spring song ‘for the Year-Feast.’”
https://en.wikipedia.org/wiki/DionysiaCome, O Dithyrambos, Baccus, Euoi, Tarsus, ivy-crowned Bromius,come in this holy spring season. Ie Paean, come saviour, in your benevolence preserve this city with happy prosperity... whom once upon a time in ecstatic Thebes Thyone bore to Zeus, becoming the mother of a beautiful child. And all the immortals danced and all the mortals rejoiced, Bacchus at your birth.
Notice all these saviors have celebrations in the winter or around the time of the winter solstice and also the spring. Interesting that we just so happen to have holidays for Jesus in the winter and the spring.This festival was held probably from the 10th to the 16th of the month Elaphebolion (the lunar month straddling the vernal equinox, i.e., Mar.-Apr in the solar calendar), three months after the rural Dionysia, probably to celebrate the end of winter and the harvesting of the year's crops.