Throughout the year, the OTO celebrates and commemorates several days it considers Holy. These days are set forth in The Book of the Law (Chapter II):
35. Let the rituals be rightly performed with joy & beauty!
36. There are rituals of the elements and feasts of the times.
37. A feast for the first night of the Prophet and his Bride!
38. A feast for the three days of the writing of the Book of the Law.
39. A feast for Tahuti and the child of the Prophet— secret, O Prophet!
40. A feast for the Supreme Ritual, and a feast for the Equinox of the Gods.
41. A feast for fire and a feast for water; a feast for life and a greater feast for death!
42. A feast every day in your hearts in the joy of my rapture!
43. A feast every night unto Nu, and the pleasure of uttermost delight!
Aleister Crowley expands upon these verses in his commentaries to The Book of the Law, and so we have the following:
The Rituals of the Elements and Feasts of the Times are observed at the Equinoxes and Solstices.
The Feast for the First Night of the Prophet and His Bride is observed on August 12.
The Feast for the Three Days of the Writing of the Book of the Law is observed on April 8, 9 and 10, beginning at noon on each day.
The Feast for the Supreme Ritual (the Invocation of Horus) is observed on March 20.
The Feast for the Equinox of the Gods is held on the northern-hemisphere Vernal Equinox of each year to commemorate the founding of Thelema in 1904. It marks the beginning of the Thelemic year.
Birth is celebrated in a Feast for Life; puberty is celebrated in a Feast for Fire (for a boy), or a Feast for Water (for a girl); and the death of the individual is commemorated in a Greater Feast for Death.
Ultimately, they are an injunction to celebrate joy & beauty in all life if we are to dissolve into the eternal ecstasy of Nu.