Send a Card to a Friend Day is observed on February 07, 2018. Just send a card to a friend on this day do stay in touch. The day was invented by a card company. It is not important if the card you send is self-made or not. The only thing that counts is that you actually send it to a friend.
It would be best to choose a friend you have not seen in a while. But certainly your other friends would be happy about a neat surprise in their letter box, too. A postcard is so much nicer than bills and advertisements. You can by the way send several postcards to several friends on Send a Card to a Friend Day. Maybe you might also like to send a postcard to someone you like to become friends with in the future.