Rule 901.5

Once all players have kept their opening hands and used the abilities of cards that allow them to take an action with those cards from their opening hands, the starting player moves the top card of his or her planar deck off that planar deck and turns it face up. If it’s a plane card, that card is the starting plane. If it’s a phenomenon card, the player puts that card on the bottom of his or her planar deck and repeats this process until a plane card is turned face up. (See rule 103.6.) No abilities of any card turned face up this way trigger during this process.

Find this number in the current Rules