Crowley identifies the Supreme Ritual as the
Invocation of Horus accorrding to the divine Vision of W., the Seer. This is the ritual that Crowley used to inaugurate the Equinox of the Gods in 1904.
This invocation can be found Liber Aba - part 4, chapter 6. (The Equinox of the Gods)
Or here:
http://hermetic.com/crowley/equinox-of-the-gods/eqotg6.html
If you fancy having a go, the appropriate date is the 20th of March. The ritual itself is fairly simple, and those of you who have recently been reading Rodney Orheus will instantly spot the method in the madness.
Call on god. Call in god. You are god.
The Feast for the Equinox of the Gods. For Crowley this meant the celebration of the Golden Dawn Equinox ceremony at the spring and autumnal equinox. It's usually around the 21st March/September. But the date does vary slightly from year to year. It can be found in the same chapter as the Supreme ritual. It is also re-printed in Liber Aba, appendix VIII and Regardie's Golden Dawn.
At first sight it may look a bit daunting. But the ritual can be easily be adapted for small groups or even solo working. If you don't like the look of it there is nothing stopping anyone making up something of their own to celebrate the changing of the solar tide.