That sounds like something you just made up, I can't find where it says any of that in the rules.
Rules, page 15: 10.0 Opportunity Cards wrote:
Opportunity Fortune cards may be held by a Player and can be played at any time during the game.(emphasis in the rule book)
Rules, page 9: 2.4 Play Cards wrote:
Cards with a green lightning symbol are Opportunity cards and may be played at any time during any phase except during the Draw phase
"At any time during any phase"