The highest of all holidays in the Satanic religion is the date of one's own birth. Every man is a god if he chooses to recognize himself as one. So, the Satanist celebrates his own birthday as the most important holiday of the year. Despite the fact that some of us may not have been wanted, or at least were not particularly planned, we're glad, even if no one else is, that we're here! You should give yourself a pat on the back, buy yourself whatever you want, treat yourself like the king (or god) that you are, and generally celebrate your birthday with as much pomp and ceremony as possible. After one's own birthday, the two major Satanic holidays are Walpurgisnacht and Halloween (or All Hallows' Eve). The solstices and equinoxes are also celebrated as holidays, as they herald the first day of the seasons.
Anton LaVey