2016-04-08 : A card with no mana cost, such as a land, has a converted mana cost of 0.

2016-04-08 : If a card in your graveyard has {X} in its mana cost, X is considered to be 0.

2016-04-08 : The phrase “different converted mana costs” compares the mana costs of cards in your graveyard to one another, not to Seasons Past. You may return a card with a converted mana cost of 6.

2016-04-08 : You choose which cards to return as Seasons Past resolves, not as you cast it.