If this card is Normal or Special Summoned: You can reveal your entire hand, and if you revealed a card that mentions "Light and Darkness Ritual", you can draw 3 cards, then discard 2 cards, also for the rest of this turn, you can only Special Summon once from the Extra Deck. You can Tribute this card; Special Summon 1 Ritual Monster that mentions "Light and Darkness Ritual" from your hand. You can only use each effect of "Celtic Mystic" once per turn.
