The most important holiday in Satanism is one's own birthday, as it is the birthdate of one's own god. This is as a reminder that to a Satanist, you are the most important being in the universe, and to honor your own vital existence and your life. LaVey recommends that a Satanist celebrate their own birthday in any way they choose, with as much pomp and ceremony as they see fit. The Satanic celebration of the birthday can also be seen as a mockery of the holidays commemorating the birth of various gods or saints in other religions.
Three Satanic holidays are named by Anton LaVey in The Satanic Bible but are not considered sacred.
One among these holidays is Walpurgisnacht, which in addition to the occult significance the date carries, also marks the formation of the Church of Satan in the year 1966, or I Annos Satanas. This date is commonly celebrated by Satanists with private or group rituals, and private parties or family celebrations to commemorate the foundation of the Church of Satan.
LaVey also mentions the summer and winter solstices, and the spring and fall equinoxes as lesser holidays. These are likewise often celebrated by ritual or private party; however, they are sometimes also used to substitute popular holidays that Satanists wish to avoid imparting a Christian overtone to but still wish to celebrate in some form.
Halloween is very commonly celebrated by Satanists, but typically there is far less occult significance attached to the date by Satanists than the public might imagine. Halloween is a popular date for both private and group ritual ceremonies, but also a popular date for Satanists to hold private parties for no other purpose than to enjoy the dark fun that is commonly celebrated on that date by the public at large. If anything, Satanists seem to take a sense of irony and humor in the holiday.
Satanism does not specifically forbid the celebration of any holidays or festivals held by other cultures or even other religions. Entirely secular holidays are commonly celebrated by Satanists either for personal reasons or merely as an excuse to have a good time. It is not entirely uncommon for Satanists to even celebrate overtly Christian holidays such as Christmas, though the religious trappings are generally stripped from the holiday, secularizing it. Many Satanists, however, either transfer such holidays as Christmas to the Winter Solstice and either place a darker spin on it or secularize it entirely, or decline to celebrate such holidays altogether.
Of recent note, June 6th, 2006 marked a Satanic High Mass in Hollywood, California by the Church of Satan. This celebration was by invitation only and limited to 100 attendees, and was held in large part to mock the superstitious fear of the date by the public. The date 06/06/06 does not hold special religious significance in Satanism, nor does the number 666. The event was documented, and many members of the Church of Satan were interviewed, by the BBC with permission.