Remus kept all the cards his friends and family had given him on various accounts such as birthdays, graduation from Hogwarts and back when he had completed his first Order Mission.
Most of them were Muggle cards given to him by his mother and Sirius (since Siri preferred static pictures on the cards rather than the wizarding version).
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A particular favorite of his was a hand drawn card having a picture of a giant chocolate bar with 20 candles, given to him by none other than Sirius Black. On the inner page of the card, it read:
Happy Birthday to my dear Moony. You shine bright like a full moon upon my dark night skies. And I can never imagine life without such a sincere friend as you. Wish I could bring all the chocolates in the world to you Moon Moon but I guess that would be labelled as impossible. However, I'll try my best to fetch the perfect gift for my perfect friend.
But unfortunately Sirius couldn't give him the gift soon, since he was sent on an Order mission shortly afterwards where the squad including him was attacked by Death Eaters. As a result of which, Siri and his fellows were badly injured and had to go in hiding for a while, so as not to blow up the Order's cover.
Soon as Sirius was released from the temporary grounding, he had gone to Remus first of all, with a box of chocolates and a Muggle mystery novel that Rem had once mentioned as a book he really wanted to read.