When you have to discard an event card, the game allows you to play any of your event cards before discarding, provided you obey the cards' timing.If the card says "Play Immediately", it is required to be played for that effect as soon as it is drawn. If it says "play immediately or at some such time" and you declined the immediate option, you can thereafter only play it at the time specified.
For an event card whose timing is "play immediately or at some such time", would you allow your players to play that card during a discard that is not happening at the specified time? I would not. But I can see people arguing that the card's timing allows them to play it immediately.