Metegeitnion

4* - Herakleia
Herakles | Feast and athletic festival for the men of Athens in the gymnasium

7 - Sacrifice to Apollon Patroos, Leto, Artemis, and Athene

12 - Sacrifice to Apollon Lykeios, Demeter, Zeus Polieus, and Athene Polias

15–18* - Eleusinia
Demeter & Persephone | Agricultural festival in thanks for the gift of corn, including athletic, horseriding, and music competitions

16 - Sacrifice to Kourotrophos and Artemis Hekate in Hekate's sancturary

19 - Sacrifice to the Heroines

20 - Sacrifice to Hera Telkhinia (of Charm)

25 - Sacrifice to Zeus Epopetes (the Overseer)

Pyanepsion

6* - Proerosia
Demeter | Agricultural cereal-grain festival

7 - Pyanepsia and Oskhophoria
Apollon Pythios | Thanksgiving to Pythian Apollo for establishing the Proerosia festival
Dionysos |  Wine-pressing festival

8* - Theseia | Theseian Games
Theseus | Athletics festival in honor of the hero Theseus

9 - Stenia
Demeter & Persephone | A women's festival connected to the Thesmophoria

11–13 - Thesmophoria
Demeter Thesmophoros | 3-day agricultural festival for married women

14 - Sacrifice to the Heroines

16 - Sacrifice to Zeus and Zeus Horios (of the Boundary)

19–21* or 26–28* - Apaturia
Zeus Phratrios & Athene Phratria | Annual family reunion to introduce new wives and babies

30 - Khalkeia
Athene | The day the loom was set up for Athene's annual peplos

Poseideon

? - Rural Dionysia
Dionysos | A small local festival for Dionysos involving drinking, local games, and theatrical performances

8* - Posideia
Poseidon Asphaleios & Poseidon Orthosios | Obscure festival involving a procession, athletic competitions, and a traditional cake offering

16 - Sacrifice to Zeus Horios

26 - Haloa
Demeter | A women's festival and feast of many foods and offerings of phallus-shaped cakes

Anthesterion

2 - Sacrifice to Dionysos

11–13 - Anthesteria
Dionysos | Wine opening festival

20–26* - The Lesser Mysteries
Pherrephata (Persephone) & Persephone Hagne | Prepatory initiation for the Greater Mysteries with a focus on the myths

23 - Diasia
Zeus Meilichios | A vegetarian family picnic outside the city walls with toy-sellers for the children

Mounichion

4 - Eros festival
Eros | Nothing is known about this festival, other than the date and the god honoured. As a spring time event it may relate to the fertility of crops. The inscription was found in the temple of Aphrodite.

4* - Adonia festival
Adonis | A private women's revelry on the rooftops with "Gardens of Adonis" (pots of lettuce with a little Adonis doll in the center)

6 - Delphinia
Apollon & Artemis (?) |  A procession of maidens carrying olive boughs wrapped in white wool to the Delphinion sanctuary in supplication

16 - Mounichia
Artemis Potnia Theron | Bear festival where offerings of "shining all round" cakes were given

19 - Olympieia
Zeus Olympos | A stream of athletic and horse-riding competitions followed by a large-scale beef feast

20 - Sacrifice to Leukaspis (khthonic hero)

21 - Sacrifice to the Tritopatores "third-fathers" (Ancestors)

Skirophorion

3* - Arrephoria/Arrhephoria
Athene | Private rite of the "dew-carriers" where her hierai conclude their service by carrying the sacred things to an underground spring

12 - Skira/Skiraphoria
Athene Skiras & Demeter | A women's festival to retrieve the piglets from the gorge; Men held an athletics competition

14 - Dipolieia
Zeus Polieus |  An "ox-killing" rite in which the sacrificial ax is tried for murder

Last Third of the Month - Diisoteria
Zeus Soter | A large-scale procession, military sailing contest, and a banquet in which the god was believed to be in attendance

Last Day of the Month - Sacrifice to Zeus Soter (the Savior) and Athene Soteria (the Savior)

* speculative date
n. nth hemisphere months (approximate)
s.  sth hemisphere months (approximate)

Note: Not all Athenian festivals are related to a season, some relate to anniversaries of important events instead