You must still pay all costs and follow all normal timing rules when casting spells from among the cards in the booster pack.

Opening Ceremony has four lines of flavor text.

You can use any unopened, official Magic booster pack, including promotional booster packs.

The spells you cast this way are cast from outside the game. Casting a card brings it into the game. After the spell leaves the stack, it will go to the battlefield, the graveyard, exile, or wherever it’s supposed to go depending on what type of spell it is. Remove the card from your deck after the game ends.