Two postcards sent by Laura (the UK).
Keep Calm and Carry On is a catchphase that originally appeared on a World War II-era British public safety poster. You can read a bit more about that here and here. After the rediscovery of the poster in 2000, a lot of funny parodies have appeared with altered text, colours and iconography. You can find some of them here. And, if you feel creative, you can even create your own poster in the Keep Calm-O-Matic.