During your turn any time you could cast a sorcery you may roll the planar die. You can do this multiple times in the same turn. To roll the die you must pay an amount of mana equal to the number of times you've already rolled the die this turn. So the first roll is free the second roll costs 1 the third roll costs 2 and so on. Rolling the die happens immediately (no one can respond to it) but any ability that triggers as a result goes on the stack and can be responded to like other triggered abilities.