Historical Rules · 30 September 2016
Rule 702.56b
Official source text from the 30 September 2016 Rules release
A forecast ability may be activated only during the upkeep step of the card’s owner and only once each turn. The controller of the forecast ability reveals the card with that ability from his or her hand as the ability is activated. That player plays with that card revealed in his or her hand until it leaves the player’s hand or until a step or phase that isn’t an upkeep step begins, whichever comes first.