In the same way that Google Now peaks into your personal correspondence to remind you about your airline and hotel reservations, it peruses the same applications to see what exciting events you have planned, and when they will be taking place. For example, if Google sees a thread in your Gmail about a day at the beach, it will send you a card to remind you about it.
If this feels like Google reaching into your underwear drawer, you do have the ability to stop the snooping. But if you are a forgetful sort who forgets every event you have planned, Google Now will gladly remind you for as long as you let it.
source: AndroidPolice via Engadget